Aquatic animal veterinarians in clinical practice, by nature of their specialty, often must select diagnostic procedures that will provide the most beneficial and cost-effective treatment. A familiar course is to establish a list of differential diagnoses during the clinical examination or necropsy. Gross lesion recognition is an art as much as a science. Vast zones of grey may confront the investigator when considering what the lesion is, what the cause could be, and what to do with it. And nobody is good at all of it: A seasoned livestock pathologist could clear an entire day’s necropsies without the help of a single histology slide. A beekeeper would immediately recognize the mites that parasitize his colony. A dog-cat pathologist or a companion animal clinician may not have a clue about gross lesions in livestock or bees. Historically, our attempts at accurate diagnosis by gross lesion recognition have been a humbling experience, and that has been the impetus for all further diagnostic specialties. Gross lesions, or lack thereof, are the first visual indication of what may be wrong with the patient. These are the lumps, the effusions, the asymmetrical oddities, the discolorations, the odors of disease that stimulate a “scientific” thought process culminating in a list of differential diagnoses. The purpose of this 2-hour interactive workshop is to present clear and not-so-clear images of common and not-so-common gross lesions in live and necropsy specimens, and in a participatory manner, establish a differential diagnosis and means for establishing a definitive diagnosis.