Inhibitor of apoptosis proteins (IAPs) constitute a family of apoptosis-suppressing proteins that contain at least one copy of a conserved domain called baculoviral IAP repeat (BIR), a zinc-binding fold involved in protein interactions. Humans and other mammals contain multiple genes encoding IAP family members, providing a diversity of variants with both common and specialized functions. IAPs are known for their ability to bind certain caspases, which are proteases responsible for apoptosis. Several IAPs contain RING (really interesting new gene) domains that bind ubiquitin-conjugating enzymes (UBC), whereas others possess UBC catalytic domains. These attributes endow many IAPs with E3 ligase activity, implicating them in the ubiquitinylation and proteasome-dependent degradation of a variety of cellular substrates. In addition, several IAP family members have multifaceted functions as platforms for coordinating signal transduction events associated with activation of particular protein kinases. Finally, some IAPs have dual functions as regulators of cell death and cell division. In this chapter, we provide an overview of IAP family proteins, including their structures and domain organizations, biochemical and cellular functions, intracellular locations, post-translational modifications, and relevance to disease.