Zoological institutions manage animals for conservation, education, entertainment, and research purposes. Zoological staff have a responsibility to safeguard the welfare of animals in their care. Retrospective morbidity and/or mortality studies (MMSs) can be useful tools to highlight common diseases in captive wildlife populations. There is currently no standardized methodology for conducting MMSs. Variation in the methodology of MMSs, particularly the categorization of diseases, can make comparisons between studies challenging and may limit the applicability of the results. A Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) compliant systematic review was performed, which identified 67 MMSs describing 146 species of captive wildlife. These MMSs are becoming more common and were predominantly performed on mammals (76/146). Prospective authors are encouraged to perform MMSs on amphibians, birds, reptiles, fish, and invertebrates. The studied animals were mostly managed at institutions in the United States of America (28/67, 41.2%) and Europe (14/67, 20.9%). Classifying individuals into age groups facilitates the identification of disease trends within age classes. Only 22/67 (32.8%) studies cited justification for their age classification; classifications should be based on a referenced source on the breeding biology of the studied species. There is variation in the body systems used by authors and into which system a disease is categorized, which makes study comparisons challenging. Diseases were predominantly categorized by etiology and body system (28/77, 36.4%). Because of its ubiquity, the use of the categorization system employed by the pathology module of the Zoological Information Management System is recommended as a useful standard. This system is imperfect, and amendments to it are suggested. The results and recommendations of this study were discussed with a panel of zoo and wildlife experts; guidelines have been formulated for prospective authors aiming to conduct MMSs in captive wildlife.