The study is based on photographs at macroscopic and histological level in order to illustrate the role of the most important pathogens and related diseases from the view of a pathological diagnosis. The reader is able to familiarize with functional anatomy, necropsy and general histology of adults and paralarvae, as well as with the identification of different pathogens and pathologies. This work is thus an invaluable guide for the diagnosis of cephalopod diseases. Besides including pathogens for non-European cephalopod species, it also provides a useful contribution encouraging marine pathologists, parasitologists, veterinarians and those involved in fishery sanitary assessments, aquarium maintenance and aquaculture practices aiming to increase their knowledge about the pathology of cephalopods.