Default reasoning is a useful inference technique which involves choosing a single context in which further inferences are to be made. If this choice is incorrect, the context may need to be switched. Dependency-directed backtracking provides a method for such context switching. Doyle’s algorithm for dependency-directed backtracking is revised to allow context switching to be guided by the calling inference system using domain knowledge. This new backtracking mechanism has been implemented as part of software for developing expert systems.