One of the most exciting developments in the field of bacterial pathogenesis in recent years is the discovery that many pathogens utilize complex nanomachines to deliver bacterially encoded effector proteins into eukaryotic and prokaryotic target cells to modulate a variety of cellular functions for the pathogen’s benefit. These protein-delivery machines include the type III secretion system (T3SS), which is widespread in nature and encoded not only by bacteria pathogenic to vertebrates or plants, but also by bacteria that are symbiotic to plants or insects. Because they are essential virulence factors for many important human pathogens, these systems are emerging as a prime target for the development of new-generation, anti-infective drugs. This book reviews our current understanding of these intriguing injection machines as well as of the closely related T3SS that serves in flagella assembly. Individual chapters focus on regulation, assembly, structure, and function of the type III secretion machine and on the evolution of the secreted effector proteins. Given its scope, this book will appeal to a broad readership, including researchers and teachers in the fields of infectious diseases, host pathogen interactions, plant and animal pathogenesis, and symbiosis.