We describe a powerful interactive debugger (Heval) embedded in a larger programming environment (Lispedit). In Lispedit, the programmer creates, edits and files Lisp functions from a uniform editor interface that makes use of the structure of Lisp expressions. With Heval, the programmer can execute Lisp expressions and functions in a very natural way through the editor interface. We describe the debugging interface and how it has evolved over several years of use. © 1983, ACM. All rights reserved.