The main topic of the thesis is a discussion of the contraction of esse (-st < est , -'s < -es), a widespread phenomenon in Latin comedy, generally known as ‘prodelision’. The thesis collects evidence for contracted forms from a large variety of sources (e.g. inscriptions, Sabellian languages and ancient grammarians) and analyses their transmission in the manuscript tradition of Latin authors. Contracted forms appear to be widespread but are neglected by or puzzling to editors. They are not misspellings, abbreviations, or representations of a sandhi phenomenon related to elision (‘prodelision’), but are clitic forms of the verb esse, showing phonetic reduction. The thesis discusses their linguistic significance, relying in particular on analysis of their behaviour in Terence: auxiliary or copula esse is clitic and its standard position seems to be clause-final, attached to the participle or predicate noun or adjective. Second, the thesis analyses another complex phenomenon of early Latin, the prosodical omission of final -s before a consonant (sigmatic ecthlipsis), relying on a database of all Terence's lines potentially involving it. Metrical evidence appears to be limited to cases in which a participle or predicate noun or adjective precedes a form of esse beginning with s-, thus confirming the prosodical distinctiveness of such a sequence. The evidence of sigmatic ecthlipsis thus ties in closely with that of contraction. The findings of the thesis have philological and editorial implications. First, they throw light on orthographic, phonetic, prosodic, and syntactic aspects of the verb ‘to be’ in Latin. It has a strong bond with the participle and predicate noun or adjective and may either be reduced phonetically in combination with such hosts, or participate in a simplification of -ss- which it may share with its host. Second, transmitted contracted spellings or readings betraying a misunderstanding of them should be given full consideration by editors and not disregarded. Contracted spellings might be restored in other conditions since manuscripts are often not trustworthy when transmitting uncontracted forms. Contracted forms may be stylistically motivated. They occur in comedy particularly in spoken metres, and in late Latin become a spelling archaism. Finally, sigmatic ecthlipsis in Terence occurs for certain only in restricted conditions: the editorial practice of printing it wherever it is possible even if not necessary is misleading.