diff --git a/flang/include/flang/Lower/AbstractConverter.h b/flang/include/flang/Lower/AbstractConverter.h
--- a/flang/include/flang/Lower/AbstractConverter.h
+++ b/flang/include/flang/Lower/AbstractConverter.h
@@ -9,11 +9,127 @@
 #ifndef FORTRAN_LOWER_ABSTRACTCONVERTER_H
 #define FORTRAN_LOWER_ABSTRACTCONVERTER_H
 
-namespace Fortran::lower {
+#include "flang/Common/Fortran.h"
+#include "mlir/IR/Module.h"
 
-// temporary stub
-class AbstractConverter;
+namespace Fortran {
+namespace common {
+template <typename>
+class Reference;
+}
+namespace evaluate {
+struct DataRef;
+template <typename>
+class Expr;
+class FoldingContext;
+struct SomeType;
+} // namespace evaluate
 
-} // namespace Fortran::lower
+namespace parser {
+class CharBlock;
+}
+namespace semantics {
+class Symbol;
+}
+
+namespace lower {
+namespace pft {
+struct Variable;
+}
+
+using SomeExpr = Fortran::evaluate::Expr<Fortran::evaluate::SomeType>;
+using SymbolRef = Fortran::common::Reference<const Fortran::semantics::Symbol>;
+class FirOpBuilder;
+
+//===----------------------------------------------------------------------===//
+// AbstractConverter interface
+//===----------------------------------------------------------------------===//
+
+/// The abstract interface for converter implementations to lower Fortran
+/// front-end fragments such as expressions, types, etc. to the FIR dialect of
+/// MLIR.
+class AbstractConverter {
+public:
+  //===--------------------------------------------------------------------===//
+  // Symbols     
+  //===--------------------------------------------------------------------===//
+
+  /// Get the mlir instance of a symbol.
+  virtual mlir::Value getSymbolAddress(SymbolRef sym) = 0;
+
+  //===--------------------------------------------------------------------===//
+  // Expressions
+  //===--------------------------------------------------------------------===//
+
+  /// Generate the address of the location holding the expression, someExpr
+  virtual mlir::Value genExprAddr(const SomeExpr &,
+                                  mlir::Location *loc = nullptr) = 0;
+  /// Generate the address of the location holding the expression, someExpr
+  mlir::Value genExprAddr(const SomeExpr *someExpr, mlir::Location loc) {
+    return genExprAddr(*someExpr, &loc);
+  }
+
+  /// Generate the computations of the expression to produce a value
+  virtual mlir::Value genExprValue(const SomeExpr &,
+                                   mlir::Location *loc = nullptr) = 0;
+  /// Generate the computations of the expression, someExpr, to produce a value
+  mlir::Value genExprValue(const SomeExpr *someExpr, mlir::Location loc) {
+    return genExprValue(*someExpr, &loc);
+  }
+
+  /// Get FoldingContext that is required for some expression
+  /// analysis.
+  virtual Fortran::evaluate::FoldingContext &getFoldingContext() = 0;
+
+  //===--------------------------------------------------------------------===//
+  // Types
+  //===--------------------------------------------------------------------===//
+
+  /// Generate the type of a DataRef
+  virtual mlir::Type genType(const Fortran::evaluate::DataRef &) = 0;
+  /// Generate the type of an Expr
+  virtual mlir::Type genType(const SomeExpr &) = 0;
+  /// Generate the type of a Symbol
+  virtual mlir::Type genType(SymbolRef) = 0;
+  /// Generate the type from a category
+  virtual mlir::Type genType(Fortran::common::TypeCategory tc) = 0;
+  /// Generate the type from a category and kind
+  virtual mlir::Type genType(Fortran::common::TypeCategory tc, int kind) = 0;
+  /// Generate the type from a Variable
+  virtual mlir::Type genType(const pft::Variable &) = 0;
+
+  //===--------------------------------------------------------------------===//
+  // Locations
+  //===--------------------------------------------------------------------===//
+
+  /// Get the converter's current location
+  virtual mlir::Location getCurrentLocation() = 0;
+  /// Generate a dummy location
+  virtual mlir::Location genLocation() = 0;
+  /// Generate the location as converted from a CharBlock
+  virtual mlir::Location genLocation(const Fortran::parser::CharBlock &) = 0;
+
+  //===--------------------------------------------------------------------===//
+  // FIR/MLIR
+  //===--------------------------------------------------------------------===//
+
+  /// Get the OpBuilder
+  virtual Fortran::lower::FirOpBuilder &getFirOpBuilder() = 0;
+  /// Get the ModuleOp
+  virtual mlir::ModuleOp &getModuleOp() = 0;
+  /// Get the MLIRContext
+  virtual mlir::MLIRContext &getMLIRContext() = 0;
+  /// Unique a symbol
+  virtual std::string mangleName(const Fortran::semantics::Symbol &) = 0;
+  /// Unique a compiler generated identifier. A short prefix should be provided
+  /// to hint at the origin of the identifier.
+  virtual std::string uniqueCGIdent(llvm::StringRef prefix,
+                                    llvm::StringRef name) = 0;
+
+  virtual ~AbstractConverter() = default;
+};
+
+} // namespace lower
+} // namespace Fortran
 
 #endif // FORTRAN_LOWER_ABSTRACTCONVERTER_H