Adipocyte complement-related protein of 30 kDa (ACRP30) is a secreted serum protein expressed exclusively in differentiated adipocytes. Recent studies have indicated that its expression and serum levels are reduced in humans and animals with obesity and insulin resistance. Metabolic studies have demonstrated a role for ACRP30 in the regulation of glucose and lipid homeostasis. This review will describe the current literature on the biochemistry of ACRP30 and its physiological functions. We will also discuss issues that are relevant to the directions of future research.