Clusterin is an enigmatic glycoprotein with a nearly ubiquitous tissue distribution and an apparent involvement in biological processes ranging from mammary gland involution to neurodegeneration in Alzheimer's disease. Its major form, a 75-80 kDa heterodimer, is secreted and present in physiological fluids, but truncated forms targeted to the nucleus have also been identified. Upregulation of clusterin mRNA and protein levels detected in diverse disease states and in in vitro systems have led to suggestions that it functions in membrane lipid recycling, in apoptotic cell death, and as a stress-induced secreted chaperone protein, amongst others. Recent studies of knockout mice have further complicated the picture by implicating clusterin in exacerbating neuronal death in hypoxia-ischemia. The question of whether clusterin is a multifunctional protein, or deploys a single primary function influenced by cellular context, remains a central issue continuing to stimulate interest in this unusual molecule.