This chapter provides an introduction to important marine pathogens, providing an overview of the diversity of pathogen types and how they affect different hosts in the marine environment. The chapter focuses on wild and cultured species and highlights that single infections are relatively rare, with co- and secondary infections being commonplace. The authors highlight the importance of understanding “normal” host tissue structure prior to interpreting pathological changes and outline the role of histology to assess pathogenicity of emerging diseases, linking presence of individual pathogens and co-infections with degree of host response. Fact sheets focus on the pathology (i.e., interaction of a specific pathogen group with the host cell/system) with high-quality histology and TEM images, emphasizing tissue changes caused by pathogens, and point the reader to presumptive diagnosis via histology while highlighting the need for confirmatory testing via other means. The pathobiome concept is introduced and explained, and the utility for predicting outcomes at the individual and population levels discussed.