Spanish infinitive sentences are independent syntactic structures whose verb is an infinitive. Although they are very common in popular speech, most grammar books do not even mention them; there is no study extant which deals with them exclusively or analyzes them as sentences; and no accurate classification of them exists. Studies which include infinitive sentences are of two types: (1) those that do not consider them sentences because the notion sentence requires a verb capable of expressing person and number information; and (2) those that treat them as sentences but without analyzing them, that is, without saying what makes them sentences or what the requirements for sentencehood are. In both types, classification is arbitrary and idiosyncratic. Two current analyses of these structures, one structural and one generative, are shown to be inadequate in accounting for them. Under the structural analysis, these sentences are treated as dependent structures whose "marked" intonation permits them to function independently. As to the generative analysis, it treats them as subordinated structures whose embedding sentences--either higher performatives or reconstructed matrices based on discourse--are deleted. Both analyses are shown to be ad hoc and devoid of empirical content. A study is needed that can determine whether these structures are sentences and that can classify them rigorously and precisely. This dissertation offers a (language-particular) syntactic-composition analysis. Proposing new definitions for verb, sentence, and other related notions which are free of the problems that have beset previous studies, it shows that infinitives are capable of expressing person, number and other notions, and that infinitive sentences are sentences. It also classifies those sentences formally--by means of intonation--and presents spectrographic evidence demonstrating that they are grouped into discrete classes on the basis of purely formal features. The formal classification shows that infinitive sentences are as systematically related as are non-infinitives, thereby achieving two important generalizations: a unified treatment of all Spanish verbs and all Spanish sentences.