Edebug is a source-level debugger for Emacs Lisp programs, with which you can:

Step through evaluation, stopping before and after each expression.
Set conditional or unconditional breakpoints.
Stop when a specified condition is true (the global break event).
Trace slow or fast, stopping briefly at each stop point, or at each breakpoint.
Display expression results and evaluate expressions as if outside of Edebug.
Automatically re-evaluate a list of expressions and display their results each time Edebug updates the display.
Output trace information on function calls and returns.
Stop when an error occurs.
Display a backtrace, omitting Edebug’s own frames.
Specify argument evaluation for macros and defining forms.
Obtain rudimentary coverage testing and frequency counts.

The first three sections below should tell you enough about Edebug to start using it.

• Using Edebug		Introduction to use of Edebug.
• Instrumenting		You must instrument your code in order to debug it with Edebug.
• Modes		Execution modes, stopping more or less often.
• Jumping		Commands to jump to a specified place.
• Misc		Miscellaneous commands.
• Breaks		Setting breakpoints to make the program stop.
• Trapping Errors		Trapping errors with Edebug.
• Views		Views inside and outside of Edebug.
• Eval		Evaluating expressions within Edebug.
• Eval List		Expressions whose values are displayed each time you enter Edebug.
• Printing in Edebug		Customization of printing.
• Trace Buffer		How to produce trace output in a buffer.
• Coverage Testing		How to test evaluation coverage.
• The Outside Context		Data that Edebug saves and restores.
• Edebug and Macros		Specifying how to handle macro calls.
• Options		Option variables for customizing Edebug.