This book investigates the historical paths leading from pronouns to markers of verbal agreement and proposes a unified formal account of this grammaticalization process. In opposition to beliefs widely held in the literature, it is argued that new agreement formatives can be coined in a multitude of syntactic environments. Still, the individual paths toward agreement are shown to exhibit a set of underlying similarities which are attributed to universal principles that govern the reanalysis of pronominal clitics as exponents of verbal agreement across languages. It is claimed that syntactic principles impose only a set of necessary conditions on the reanalysis in question, while its ultimate trigger is morphological in nature. More specifically, it is argued that the acquisition of inflectional morphology is governed by blocking effects which operate during language acquisition and promote the grammaticalization of new markers if this change serves to replace 'worn-out', underspecified forms with new, more specified candidates.