In this paper program comprehension techniques are examined within the context of testing. First, the tasks of a tester are identified, then the information requirements of a tester to fulfill these tasks. Comprehension is viewed as a knowledge acquisition process. The knowledge needed depends on the level at which one is testing. For system testing other knowledge is required than for unit and integration testing. In light of the scope of testing, the paper concludes that it is the tester who needs the broadest knowledge about a software system. Having established the information requirements of testing, a set of tools are presented which help to satisfy these requirements and their practical application discussed.