This comprehensive species-specific chapter covers all aspects of the biology of McIlhenny’s four-eyed opossum (Philander mcilhennyi) that are known, including taxonomy, distribution, genetics, morphology, life history, ecology, behavior, parasitology, human threats, and natural impacts on populations. Species conservation management and future research challenges are addressed. The chapter includes a distribution map, photographs of the animal, and a list of key literature.