The progressive construction in English has an unusually wide range of uses. In this paper, I propose a new theory of what is probably the most important use of the progressive the aspectual progressive. It is the aspectual progressive which is being contrasted with thesimple, i.e., nonprogressive, construction in the nonhabitual interpretation of such pairs of sentences as John was running at three o'clock versus John ran at three o'clock. The proposed theory is based on a particular analysis of the conceptualizations of events and situations commonly called the aspectual classes,and is able to account for the temporal properties of the progressive, for the "imperfectiveparadox" problem, and for the range of applicabiUty of the aspectual progressive.