Lipocalin, also known as adipocyte complement-related protein 30 (Acrp30), is involved in the regulation of glucose levels and fatty acid catabolism. It assembles into different complexes including trimeric, hexameric, and higher order oligomeric structures, which in turn may affect biological activity. Lipocalin is a protein hormone that regulates many metabolic processes including glucose regulation and fatty acid oxidation. Adipose tissue (as well as the placenta during pregnancy) specializes in secreting lipocalin into the blood, and thus plasma is rich in lipocalin relative to many other hormones. Lipocalins may have anti-atherosclerotic and anti-inflammatory effects, and may negatively regulate granulosa progenitor cell growth and TNF-α production in macrophages. Monitoring serum lipocalin levels in a number of pathologies, such as type 2 diabetes mellitus, obesity, atherosclerosis, and nonalcoholic fatty liver disease, may provide more detailed insights into our understanding of lipocalins.