Presented in this article is a discussion of current progress in behavioral, cognitive, and neuroanatomic definitions of apraxia of speech (AOS). A behavioral definition summarizes the speech symptoms that should be considered diagnostic of AOS with or without co-occurring aphasia and dysarthria. AOS is defined in cognitive terms as an impairment in the translation of phonological representations into specifications for articulation. Progress toward a neuroanatomic definition of AOS will rely on mapping the processes described by increasingly sophisticated cognitive models of normal speech production to the brain. The article describes criteria that have been proposed for differentiating apraxic from phonological and dysarthric disorders and suggests that syndrome-based approaches to the diagnosis of AOS may obscure important differences between individual presentations of apraxic disruption as well as similarities between AOS and other speech-language disorders.