diff --git a/clang_delta/ExpressionDetector.cpp b/clang_delta/ExpressionDetector.cpp
index cf703924..22bdfcff 100644
--- a/clang_delta/ExpressionDetector.cpp
+++ b/clang_delta/ExpressionDetector.cpp
@@ -64,7 +64,8 @@ class IncludesPPCallbacks : public PPCallbacks {
                           StringRef FileName, bool IsAngled,
                           CharSourceRange FilenameRange, OptionalFileEntryRef File,
                           StringRef SearchPath, StringRef RelativePath,
-                          const Module *Imported,
+                          const Module *SuggestedModule,
+                          bool ModuleImported,
                           SrcMgr::CharacteristicKind FileType) override;
 
 private:
@@ -85,7 +86,8 @@ void IncludesPPCallbacks::InclusionDirective(SourceLocation HashLoc,
                                              OptionalFileEntryRef /*File*/,
                                              StringRef /*SearchPath*/,
                                              StringRef /*RelativePath*/,
-                                             const Module * /*Imported*/,
+                                             const Module * /*SuggestedModule*/,
+                                             bool /*ModuleImported*/,
                                              SrcMgr::CharacteristicKind /*FileType*/)
 {
   if (!SrcManager.isInMainFile(HashLoc))
@@ -118,7 +120,11 @@ bool LocalTmpVarCollector::VisitDeclRefExpr(DeclRefExpr *DRE)
   const VarDecl *VD = dyn_cast<VarDecl>(DRE->getDecl());
   if (!VD)
     return true;
+#if LLVM_VERSION_MAJOR >= 19
+  if (VD->getName().starts_with(Prefix))
+#else
   if (VD->getName().startswith(Prefix))
+#endif
     TmpVars.push_back(VD);
   return true;
 }
@@ -363,7 +369,11 @@ void ExpressionDetector::addOneTempVar(const VarDecl *VD)
 {
   if (!VD)
     return;
+#if LLVM_VERSION_MAJOR >= 19
+  if (!VD->getName().starts_with(TmpVarNamePrefix))
+#else
   if (!VD->getName().startswith(TmpVarNamePrefix))
+#endif
     return;
   if (const Expr *E = VD->getInit())
     ProcessedExprs[VD] = E->IgnoreParenImpCasts();
@@ -374,9 +384,15 @@ bool ExpressionDetector::refToTmpVar(const NamedDecl *ND)
   StringRef Name = ND->getName();
   // We don't want to repeatly replace temporary variables
   // __creduce_expr_tmp_xxx, __creduce_printed_yy and __creduce_checked_zzz.
+#if LLVM_VERSION_MAJOR >= 19
+  return Name.starts_with(TmpVarNamePrefix) ||
+         Name.starts_with(PrintedVarNamePrefix) ||
+         Name.starts_with(CheckedVarNamePrefix);
+#else
   return Name.startswith(TmpVarNamePrefix) ||
          Name.startswith(PrintedVarNamePrefix) ||
          Name.startswith(CheckedVarNamePrefix);
+#endif
 }
 
 // Reference: IdenticalExprChecker.cpp from Clang
@@ -524,8 +540,13 @@ bool ExpressionDetector::isValidExpr(Stmt *S, const Expr *E)
       if (const DeclRefExpr *SubE =
           dyn_cast<DeclRefExpr>(UO->getSubExpr()->IgnoreParenCasts())) {
         StringRef SubEName = SubE->getDecl()->getName();
+#if LLVM_VERSION_MAJOR >= 19
+        if (SubEName.starts_with(PrintedVarNamePrefix) ||
+            SubEName.starts_with(CheckedVarNamePrefix))
+#else
         if (SubEName.startswith(PrintedVarNamePrefix) ||
             SubEName.startswith(CheckedVarNamePrefix))
+#endif
           return false;
       }
     }
@@ -541,7 +562,11 @@ bool ExpressionDetector::isValidExpr(Stmt *S, const Expr *E)
       bool IsLit = SC == Stmt::IntegerLiteralClass ||
                    SC == Stmt::FloatingLiteralClass;
       if (IsLit && DRE &&
+#if LLVM_VERSION_MAJOR >= 19
+          DRE->getDecl()->getName().starts_with(TmpVarNamePrefix) &&
+#else
           DRE->getDecl()->getName().startswith(TmpVarNamePrefix) &&
+#endif
           S->getStmtClass() == Stmt::IfStmtClass) {
         return false;
       }
diff --git a/clang_delta/RemoveNamespace.cpp b/clang_delta/RemoveNamespace.cpp
index 0f8fc427..82cd3b82 100644
--- a/clang_delta/RemoveNamespace.cpp
+++ b/clang_delta/RemoveNamespace.cpp
@@ -944,7 +944,11 @@ void RemoveNamespace::handleOneNamedDecl(const NamedDecl *ND,
     TransAssert(IdInfo && "Invalid IdentifierInfo!");
     NewName += IdInfo->getName();
     // Make sure we have valid suffix for user literals
+#if LLVM_VERSION_MAJOR >= 19
+    if (IsUserLiteral && IdInfo->getName().starts_with("_")) {
+#else
     if (IsUserLiteral && IdInfo->getName().startswith("_")) {
+#endif
       NewName = "_" + NewName;
     }
     NamedDeclToNewName[ND] = NewName;
diff --git a/clang_delta/RenameCXXMethod.cpp b/clang_delta/RenameCXXMethod.cpp
index e71cc738..680c03da 100644
--- a/clang_delta/RenameCXXMethod.cpp
+++ b/clang_delta/RenameCXXMethod.cpp
@@ -426,7 +426,11 @@ bool RenameCXXMethod::isValidName(const StringRef &Name)
 {
   size_t PrefixLen = MethodNamePrefix.length();
   StringRef NamePrefix = Name.substr(0, PrefixLen);
+#if LLVM_VERSION_MAJOR >= 19
+  if (NamePrefix != MethodNamePrefix)
+#else
   if (!NamePrefix.equals(MethodNamePrefix))
+#endif
     return false;
   llvm::APInt Num;
   return !Name.drop_front(PrefixLen).getAsInteger(10, Num);
diff --git a/clang_delta/Transformation.cpp b/clang_delta/Transformation.cpp
index b350eec5..707a0209 100644
--- a/clang_delta/Transformation.cpp
+++ b/clang_delta/Transformation.cpp
@@ -95,7 +95,7 @@ void Transformation::Initialize(ASTContext &context)
 void Transformation::outputTransformedSource(llvm::raw_ostream &OutStream)
 {
   FileID MainFileID = SrcManager->getMainFileID();
-  const RewriteBuffer *RWBuf = TheRewriter.getRewriteBufferFor(MainFileID);
+  const llvm::RewriteBuffer *RWBuf = TheRewriter.getRewriteBufferFor(MainFileID);
 
   // RWBuf is non-empty upon any rewrites
   TransAssert(RWBuf && "Empty RewriteBuffer!");
diff --git a/clang_delta/TransformationManager.cpp b/clang_delta/TransformationManager.cpp
index 79b17cee..e16e07ce 100644
--- a/clang_delta/TransformationManager.cpp
+++ b/clang_delta/TransformationManager.cpp
@@ -24,6 +24,7 @@
 #include "clang/Lex/PreprocessorOptions.h"
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Parse/ParseAST.h"
+#include "llvm/Support/VirtualFileSystem.h" // For llvm::vfs::get  GlobalVirtualFileSystem()
 
 #include "Transformation.h"
 
@@ -90,8 +91,14 @@ bool TransformationManager::initializeCompilerInstance(std::string &ErrorMsg)
 
   ClangInstance = new CompilerInstance();
   assert(ClangInstance);
-  
+
+#if LLVM_VERSION_MAJOR >= 20
+  // Get the global virtual file system
+  auto VFS = llvm::vfs::getRealFileSystem();
+  ClangInstance->createDiagnostics(*VFS);
+#else
   ClangInstance->createDiagnostics();
+#endif
 
   TargetOptions &TargetOpts = ClangInstance->getTargetOpts();
   PreprocessorOptions &PPOpts = ClangInstance->getPreprocessorOpts();