The AI literature contains many definitions of diagnostic reasoning most of which are defined in terms of the logical entailment relation. We use existing work on approximate entailment to define notions of approximation in diagnosis. We show how such a notion of approximate diagnosis can be exploited in various diagnostic strategies. We illustrate these strategies by performing diagnosis in a small car domain example. 1 Motivation The AI literature contains many definitions of diagnostic reasoning. However, there are many reasons why we should not search for the appropriate definition of diagnosis, but instead search for alternative definitions, and investigate how they relate to each other. There exists a whole space of reasonable notions of diagnosis. These notions can be seen as mutual approximations. Strategies for approximate diagnosis can be used (1) to choose another, related notion of diagnosis when one definition of diagnosis fails (e.g. too many diagnoses, no diagnos...