Emydomyces testavorans is a recently-described keratinophilic fungal organism in the order Onygenales, which also includes Ophidiomyces ophidiicola, Nannizziopsis spp., and Paranannizziopsis spp. E. testavorans is associated with shell lesions in multiple species of aquatic and some terrestrial chelonians. Clinically, these lesions can present as focal or multifocal discoloration, flaking, erosions, ulcerations, and the formation of deep pitting lesions. Keratin inclusion cysts, the most characteristic lesions associated with E. testavorans infection, can expand into the coelomic cavity and compress viscera. Significant boney lesions may be present even in the absence of apparent surface disease, thus obscuring the clinical severity of infection until advanced stages. E. testavorans has been detected in multiple zoological collections, companion chelonians, and in wild populations, most notably the Northwestern pond turtle (Actinemys marmarota), in which E. testavorans-associated shell disease is considered a major conservation concern for populations in Washington State, USA. While Koch’s postulates have yet to be fulfilled and much of the biology and epidemiology of this pathogen are currently characterized, E. testavorans appears to be an important differential diagnosis for dermatomycosis in chelonians. This master class will review currently available knowledge about E. testavorans, including in vitro fungal growth characteristics and antifungal sensitivity patterns, affected species, clinical presentation and lesion progression, diagnostic methods and evidence-based sampling strategies, biosecurity and disinfection protocols, and treatment recommendations. This will provide a strong foundational knowledge base about this emerging fungal pathogen for reptile clinicians.