This paper has two goals. The first is to formulate an adequate account of the semantics of the progressive aspect in English: the semantics of Agatha is making a cake, as opposed to Agatha makes a cake. This account presupposes a version of the so-called Aristotelian classification of verbs in English into EVENT, PROCESS and STATE verbs. The second goal of this paper is to refine this classification so as to account for the infamous category switch problem, the problem of (...) how it is that modification of a verb like run by an adverbial like to the store can turn a PROCESS phrase (run) into an EVENT phrase (run to the store). Views discussed include those of Aqvist, Bach, Bennett, Bennett and Partee, Dowry, Montague and Scott, and Vendler. (shrink)