Cytology is a fundamental part of marine mammal veterinary medicine that is involved in preventive medicine programs in captive animals and in the health assessment of wild populations. Marine mammals often exhibit few clinical signs of disease; thus, the cost-effective and widely accessible nature of cytologic sampling renders it one of the most important diagnostic procedures with these species. Many of these mammals are endangered, protected, and located in developing nations in which resources may be scarce. This article can be used as a field guide to advise a veterinarian, biologist, or technician working with cetaceans or sirenians. A simplistic cost-effective staining technique is used, which is ideal for situations in which funds, facilities, or time may be a limiting factor in clinical practice.