Virulence-associated bacterial type III secretion systems are multiprotein molecular machines that promote the pathogenicity of bacteria towards eukaryotic host cells. These machines form needle-like structures, named injectisomes, that span both bacterial and host membranes, forming a direct conduit for the delivery of bacterial proteins into host cells. Once within the host, these bacterial effector proteins are capable of manipulating a multitude of host cell functions. In recent years, the knowledge of assembly, structure and function of these machines has grown substantially and is presented and discussed in this review.