diff --git a/cmd/cqlplay/main.go b/cmd/cqlplay/main.go
index 306f6f6..db19ccd 100644
--- a/cmd/cqlplay/main.go
+++ b/cmd/cqlplay/main.go
@@ -27,6 +27,7 @@ import (
 	"time"
 
 	"flag"
+
 	log "github.com/golang/glog"
 	"github.com/google/cql"
 	"github.com/google/cql/retriever/local"
@@ -107,7 +108,13 @@ func handleEvalCQL(w http.ResponseWriter, req *http.Request) {
 		return
 	}
 
-	elm, err := cql.Parse(req.Context(), []string{evalCQLReq.CQL, fhirHelpers}, cql.ParseConfig{DataModels: [][]byte{fhirDM}})
+	// Combine main CQL with additional libraries and FHIRHelpers
+	cqlInputs := append([]string{evalCQLReq.CQL}, evalCQLReq.Libraries...)
+	// TODO: now that users can supply their own libraries, we may wish to only add FHIRHelpers if
+	// it's not already included. Though, our parser really only works with FHIR Helpers 4.0.1.
+	cqlInputs = append(cqlInputs, fhirHelpers)
+
+	elm, err := cql.Parse(req.Context(), cqlInputs, cql.ParseConfig{DataModels: [][]byte{fhirDM}})
 	if err != nil {
 		sendError(w, fmt.Errorf("failed to parse: %w", err), http.StatusInternalServerError)
 		return
@@ -147,8 +154,9 @@ func sendError(w http.ResponseWriter, err error, code int) {
 }
 
 type evalCQLRequest struct {
-	CQL  string `json:"cql"`
-	Data string `json:"data"`
+	CQL       string   `json:"cql"`
+	Data      string   `json:"data"`
+	Libraries []string `json:"libraries"`
 }
 
 func getTerminologyProvider() (*terminology.LocalFHIRProvider, error) {
diff --git a/cmd/cqlplay/main_test.go b/cmd/cqlplay/main_test.go
index 18b59ba..847fd39 100644
--- a/cmd/cqlplay/main_test.go
+++ b/cmd/cqlplay/main_test.go
@@ -16,7 +16,6 @@ package main
 
 import (
 	"encoding/json"
-	"fmt"
 	"io"
 	"net/http"
 	"net/http/httptest"
@@ -32,6 +31,7 @@ func TestServerHandler(t *testing.T) {
 		name       string
 		cql        string
 		data       string
+		libraries  []string
 		bodyJSON   string
 		wantOutput string
 	}{
@@ -86,6 +86,38 @@ func TestServerHandler(t *testing.T) {
 				}
 			]`,
 		},
+		{
+			name: "CQL with multiple libraries",
+			cql: dedent.Dedent(`
+			library Main version '1.0.0'
+			include HelperLib version '1.0.0' called Helper
+			define result: Helper.ConstantValue + 10`),
+			libraries: []string{dedent.Dedent(`
+			library HelperLib version '1.0.0'
+			define public ConstantValue: 42`)},
+			wantOutput: `[
+				{
+					"libName": "HelperLib",
+					"libVersion": "1.0.0",
+					"expressionDefinitions": {
+						"ConstantValue": {
+							"@type": "System.Integer",
+							"value": 42
+						}
+					}
+				},
+				{
+					"libName": "Main",
+					"libVersion": "1.0.0",
+					"expressionDefinitions": {
+						"result": {
+								"@type": "System.Integer",
+								"value": 52
+							}
+					}
+				}
+			]`,
+		},
 	}
 
 	for _, tc := range tests {
@@ -97,7 +129,23 @@ func TestServerHandler(t *testing.T) {
 			server := httptest.NewServer(h)
 			defer server.Close()
 
-			bodyJSON := fmt.Sprintf(`{"cql": %q, "data": %q}`, tc.cql, tc.data)
+			// Build the request body using anonymous struct
+			reqBody := struct {
+				CQL       string   `json:"cql"`
+				Data      string   `json:"data"`
+				Libraries []string `json:"libraries"`
+			}{
+				CQL:       tc.cql,
+				Data:      tc.data,
+				Libraries: tc.libraries,
+			}
+
+			bodyBytes, err := json.Marshal(reqBody)
+			if err != nil {
+				t.Fatalf("json.Marshal(%v) returned an unexpected error: %v", reqBody, err)
+			}
+			bodyJSON := string(bodyBytes)
+
 			resp, err := http.Post(server.URL+"/eval_cql", "application/json", strings.NewReader(bodyJSON))
 			if err != nil {
 				t.Fatalf("http.Post(%v) with body %v returned an unexpected error: %v", server.URL, bodyJSON, err)
@@ -109,7 +157,7 @@ func TestServerHandler(t *testing.T) {
 			}
 			got := string(body)
 			if !cmp.Equal(normalizeJSON(t, got), normalizeJSON(t, tc.wantOutput)) {
-				t.Errorf("POST to /eval_cql to CQL server with body %v returned %v, want %v", tc.bodyJSON, got, tc.wantOutput)
+				t.Errorf("POST to /eval_cql to CQL server with body %s returned %v, want %v", bodyJSON, got, tc.wantOutput)
 			}
 		})
 	}
diff --git a/cmd/cqlplay/static/cqlPlay.js b/cmd/cqlplay/static/cqlPlay.js
index 802898a..bceb68f 100644
--- a/cmd/cqlplay/static/cqlPlay.js
+++ b/cmd/cqlplay/static/cqlPlay.js
@@ -39,6 +39,9 @@ context Patient`;
 
 let data = syntheticPatient;
 
+// Libraries array to store additional CQL libraries
+let libraries = [];
+
 let results = '';
 
 // Helper functions:
@@ -81,6 +84,16 @@ function bindButtonActions() {
       .addEventListener('click', function(e) {
         showDataTab();
       });
+  document.getElementById('librariesTabButton')
+      .addEventListener('click', function(e) {
+        showLibrariesTab();
+      });
+  document.getElementById('addLibrary')
+      .addEventListener('click', function(e) {
+        addLibrary();
+      });
+  document.getElementById('uploadLibrary')
+      .addEventListener('change', handleFileUpload);
 }
 
 /**
@@ -98,7 +111,15 @@ function runCQL() {
   };
   xhr.open('POST', '/eval_cql', true);
   xhr.setRequestHeader('Content-Type', 'text/json');
-  xhr.send(JSON.stringify({'cql': code, 'data': data}));
+  
+  // Collect library content to send with request
+  const libraryContents = libraries.map(lib => lib.content);
+  
+  xhr.send(JSON.stringify({
+    'cql': code, 
+    'data': data,
+    'libraries': libraryContents
+  }));
 }
 
 /**
@@ -107,9 +128,11 @@ function runCQL() {
 function showDataTab() {
   document.getElementById('cqlEntry').style.display = 'none';
   document.getElementById('dataEntry').style.display = 'block';
+  document.getElementById('librariesEntry').style.display = 'none';
 
-  document.getElementById('dataTabButton').className += 'active';
+  document.getElementById('dataTabButton').className = 'active';
   document.getElementById('cqlTabButton').className = '';
+  document.getElementById('librariesTabButton').className = '';
 }
 
 /**
@@ -118,9 +141,134 @@ function showDataTab() {
 function showCQLTab() {
   document.getElementById('cqlEntry').style.display = 'block';
   document.getElementById('dataEntry').style.display = 'none';
+  document.getElementById('librariesEntry').style.display = 'none';
 
-  document.getElementById('cqlTabButton').className += 'active';
+  document.getElementById('cqlTabButton').className = 'active';
   document.getElementById('dataTabButton').className = '';
+  document.getElementById('librariesTabButton').className = '';
+}
+
+/**
+ * showLibrariesTab shows the Libraries tab and hides other tabs.
+ */
+function showLibrariesTab() {
+  document.getElementById('cqlEntry').style.display = 'none';
+  document.getElementById('dataEntry').style.display = 'none';
+  document.getElementById('librariesEntry').style.display = 'block';
+
+  document.getElementById('librariesTabButton').className = 'active';
+  document.getElementById('cqlTabButton').className = '';
+  document.getElementById('dataTabButton').className = '';
+}
+
+/**
+ * addLibrary adds a new library to the libraries list and updates the UI.
+ */
+function addLibrary(name = '', content = '') {
+  const libraryId = Date.now(); // Unique ID for the library
+  
+  // Add to libraries array
+  libraries.push({
+    id: libraryId,
+    name: name,
+    content: content
+  });
+  
+  // Update the UI
+  renderLibraries();
+  
+  // Save to localStorage
+  saveLibrariesToLocalStorage();
+}
+
+/**
+ * removeLibrary removes a library from the libraries list and updates the UI.
+ */
+function removeLibrary(libraryId) {
+  libraries = libraries.filter(lib => lib.id !== libraryId);
+  renderLibraries();
+  saveLibrariesToLocalStorage();
+}
+
+/**
+ * renderLibraries updates the libraries UI with the current libraries.
+ */
+function renderLibraries() {
+  const container = document.getElementById('librariesContainer');
+  container.innerHTML = '';
+  
+  libraries.forEach(library => {
+    const libraryContainer = document.createElement('div');
+    libraryContainer.className = 'libraryContainer';
+    
+    const headerDiv = document.createElement('div');
+    headerDiv.className = 'libraryHeader';
+    
+    // Create label for the library name input
+    const nameLabel = document.createElement('div');
+    nameLabel.className = 'libraryNameLabel';
+    
+    const nameInput = document.createElement('input');
+    nameInput.type = 'text';
+    nameInput.value = library.name;
+    nameInput.placeholder = 'Library Name';
+    nameInput.className = 'libraryNameInput';
+    nameInput.oninput = function(e) {
+      library.name = e.target.value;
+      saveLibrariesToLocalStorage();
+    };
+    
+    nameLabel.appendChild(document.createTextNode('Library Name:'));
+    nameLabel.appendChild(nameInput);
+    
+    const removeButton = document.createElement('button');
+    removeButton.className = 'removeLibraryButton';
+    removeButton.textContent = 'Remove';
+    removeButton.onclick = function() {
+      removeLibrary(library.id);
+    };
+    
+    headerDiv.appendChild(nameLabel);
+    headerDiv.appendChild(removeButton);
+    
+    const editorDiv = document.createElement('div');
+    editorDiv.className = 'codeInputContainer';
+    
+    const codeInput = document.createElement('code-input');
+    codeInput.setAttribute('lang', 'cql');
+    codeInput.setAttribute('placeholder', 'Type CQL Library Here');
+    codeInput.className = 'codeInput';
+    codeInput.value = library.content;
+    codeInput.onchange = function(e) {
+      library.content = e.target.value;
+      saveLibrariesToLocalStorage();
+    };
+    
+    editorDiv.appendChild(codeInput);
+    
+    libraryContainer.appendChild(headerDiv);
+    libraryContainer.appendChild(editorDiv);
+    
+    container.appendChild(libraryContainer);
+  });
+}
+
+/**
+ * saveLibrariesToLocalStorage saves the libraries to localStorage.
+ */
+function saveLibrariesToLocalStorage() {
+  localStorage.setItem('cqlLibraries', JSON.stringify(libraries));
+}
+
+/**
+ * loadLibrariesFromLocalStorage loads the libraries from localStorage.
+ */
+function loadLibrariesFromLocalStorage() {
+  const storedLibraries = localStorage.getItem('cqlLibraries');
+  if (storedLibraries) {
+    libraries = JSON.parse(storedLibraries);
+    renderLibraries();
+  }
 }
 
 /**
@@ -150,6 +298,33 @@ function setupPrism() {
       'syntax-highlighted', codeInput.templates.prism(Prism, []));
 }
 
+/**
+ * handleFileUpload processes uploaded CQL library files
+ */
+function handleFileUpload(event) {
+  const fileList = event.target.files;
+  if (fileList.length === 0) {
+    return; // No file selected
+  }
+  
+  const file = fileList[0];
+  const reader = new FileReader();
+  
+  reader.onload = function(e) {
+    const content = e.target.result;
+    // Extract library name from filename (remove .cql extension)
+    const fileName = file.name.replace(/\.cql$/i, '');
+    
+    // Add the library with the file content
+    addLibrary(fileName, content);
+  };
+  
+  reader.readAsText(file);
+  
+  // Reset the file input so the same file can be selected again
+  event.target.value = '';
+}
+
 /**
  * main is the entrypoint for the script.
  */
@@ -159,8 +334,15 @@ function main() {
   bindInputsOnChange();
   bindButtonActions();
 
-  // Initially hide dataEntry tab:
+  // Load libraries from localStorage
+  loadLibrariesFromLocalStorage();
+
+  // Initially hide non-CQL tabs
   document.getElementById('dataEntry').style.display = 'none';
+  document.getElementById('librariesEntry').style.display = 'none';
+  
+  // Set CQL tab as active
+  document.getElementById('cqlTabButton').className = 'active';
 }
 
 main();  // All code actually executed when the script is loaded by the HTML.
\ No newline at end of file
diff --git a/cmd/cqlplay/static/index.html b/cmd/cqlplay/static/index.html
index ddd8a8a..ef91255 100644
--- a/cmd/cqlplay/static/index.html
+++ b/cmd/cqlplay/static/index.html
@@ -43,6 +43,7 @@ <h2> ⚡ CQL Playground </h2>
 <div class="tabholder">
 	<button  id="cqlTabButton"> CQL </button>
 	<button  id="dataTabButton"> Data </button>
+	<button  id="librariesTabButton"> Libraries </button>
 </div>
 
 <div id="cqlEntry" class="tabContent">
@@ -57,6 +58,23 @@ <h3>Data Editor</h3>
 		<code-input lang="json" placeholder="Enter synthetic JSON FHIR Bundle here." class="codeInput" id="dataInput"></code-input>
 	</div>
 </div>
+<div id="librariesEntry" class="tabContent">
+	<h3>CQL Libraries</h3>
+	<p class="instructions">
+		Add additional CQL libraries to be included in your CQL execution.
+		Libraries can be created directly or uploaded from files.
+		Each library should have a name and contain valid CQL code. <br />
+		<b>Our parser only supports FHIRHelpers 4.0.1, it is automatically included in all execution--you do not need to add it here.</b>
+	</p>
+	<div id="librariesContainer">
+		<!-- Libraries will be added here dynamically -->
+	</div>
+	<div class="libraryButtons">
+		<button id="addLibrary" class="addButton">Add Library</button>
+		<label for="uploadLibrary" class="uploadButton">Add Library from File</label>
+		<input type="file" id="uploadLibrary" accept=".cql" style="display:none">
+	</div>
+</div>
 <button id="submit" class="submitButton">
   Run!
 </button>
diff --git a/cmd/cqlplay/static/styles.css b/cmd/cqlplay/static/styles.css
index ae7d88d..2e3e4f6 100644
--- a/cmd/cqlplay/static/styles.css
+++ b/cmd/cqlplay/static/styles.css
@@ -91,4 +91,83 @@ code-input {
 
 .submitButton {
   margin: 10px;
+  font-family: Roboto, Helvetica;
+  font-size: 1em;
+  font-weight: normal;
+}
+
+.addButton {
+  margin: 10px;
+  background-color: #4CAF50;
+  color: white;
+  border: none;
+  padding: 8px 16px;
+  cursor: pointer;
+  font-family: Roboto, Helvetica;
+  font-size: 1em;
+  font-weight: normal;
+}
+
+.uploadButton {
+  margin: 10px;
+  background-color: #2196F3;
+  color: white;
+  border: none;
+  padding: 8px 16px;
+  cursor: pointer;
+  display: inline-block;
+  font-family: Roboto, Helvetica;
+  font-size: 1em;
+  font-weight: normal;
+}
+
+.libraryButtons {
+  display: flex;
+  flex-direction: row;
+}
+
+.libraryContainer {
+  margin-bottom: 20px;
+  border: 1px solid #ddd;
+  padding: 10px;
+  background-color: #f9f9f9;
+}
+
+.libraryHeader {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  margin-bottom: 10px;
+}
+
+.libraryNameLabel {
+  display: flex;
+  align-items: center;
+  font-family: Roboto, Helvetica;
+  font-weight: bold;
+}
+
+.libraryNameInput {
+  padding: 5px;
+  width: 300px;
+  margin-left: 10px;
+}
+
+.removeLibraryButton {
+  background-color: #f44336;
+  color: white;
+  border: none;
+  padding: 5px 10px;
+  cursor: pointer;
+  font-family: Roboto, Helvetica;
+  font-size: 1em;
+  font-weight: normal;
+}
+
+.instructions {
+  font-family: Roboto, Helvetica;
+  font-size: 14px;
+  color: #555;
+  margin-bottom: 15px;
+  line-height: 1.4;
 }
\ No newline at end of file