This article, which emerges from my collaboration with Alan Prince on Prosodic Morphology, will explore the consequences of the Prosodic Morphology Hypothesis for a fairly complete account of the central regularities of canonical form in two Semitic root-and-pattern morphological systems, those of Arabic and Akkadian. We will see that the core of the Arabic nominal system is templatic in character, with templates that conform to (3). But two more specialized nominal constructions depart from (3) and are provably non-templatic. They are analyzed instead in terms of two other notions from Prosodic Morphology theory, prosodic circumscription and a-templatic prosodic morphology. The Arabic and Akkadian verb system is even more radically non-templatic; just a single template underlies all verb forms, and other morphological regularities are derived by rules of affixation, sometimes via prosodic circumscription.