Assuming the existence of Aspect Phrase between NegP and VP, this paper claims that infinitival to shares [Head, AspP] with progressive -ing and perfective -en. Such an analysis predicts that the verb complements of infinitival to must be in the bare form, since to, -ing, and -en should be in complementary distribution. It also explains why infinitival to follows not in negation and primary stress never falls on infinitival to. The semantic properties of the to-infinitives may be systematically contrasted with those of the perfective form in relation to the matrix tense, confirming the aspectual nature of to-infinitives. The fact that to-infinitives and the progressive form both evolved from PPs provides historical evidence.