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 since that cannot occur with other aspects. These claims are supported by the morphosyntax and semantics of aspect particles in Scottish Gaelic, with additional data from English. I investigate six particles in Scottish Gaelic, focusing on four I claim to mark various aspects and one I claim to be simply a preposition. I argue that in addition to two inclusion aspects, perfective and imperfective (expressed via a synthetic form and by a’, respectively), Scottish Gaelic shows four distinctions of precedence aspect—two retrospective (air, as dèidh) and two prospective (gu, a’ dol do). I provide a neo-Reichenbachian analysis of these particles within event semantics. In each case, the particle is an instantiation of an Aspect head that existentially quantifies over an event and places its runtime in a relation to reference time. I also argue that the particle ann, which seems to appear with both verbal and nominal material, is not an aspect particle but a preposition. Its appearance in the same linear position as the aspect particles belies its distinct syntactic structure.

Overall, the data indicate the benefit of a view of grammatical aspect in which the basic time relations of reference time within, before, and after event time delineate groups of aspects rather than individual distinctions. This view of aspect is a more cohesive alternative to one in which aspects that may actually be very similar are taken to exist in separate categories.

Many of the data in my thesis are drawn from elicitations as part of the Arizona Scottish Gaelic Syntax Project, funded in part by the National Science Foundation (NSF), #BCS0602768A. My research was also funded in part by a pre-doctoral grant from the University of Arizona Social and Behavioral Sciences Research Institute.