This dissertation presents a theory of grammatical aspect in which perfects and prospectives form a sub-group separate from perfectives and imperfectives. I claim that aspects in this sub-group display a number of similar semantic and syntactic behaviors because of the way in which they relate event and reference times. While perfectives and imperfectives situate these times in inclusion relations, perfects and prospectives separate event time from reference time. This effectively creates an interval, homogeneous with respect to the eventuality, that can be interpreted as a state. The separation of the times in these aspects also means that modification of the interval between these times is possible, as is modification by adverbials like s...