is a fish pathogen that has become a powerful and well-established model that has accelerated our understanding of the mechanisms of mycobacterial disease. is a versatile surrogate for understanding the closely related human pathogen , which causes tuberculosis in humans. has defined key mechanisms of pathogenesis, both shared with and unique to this species. In this review, we discuss the discovery of as an occasional human pathogen, the shared aspects of pathogenesis with and how has been exploited as a model to define the molecular mechanisms of mycobacterial pathogenesis across several phases of infection.