The enzyme 5-lipoxygenase (5-LO) initiates the synthesis of leukotrienes. For this reason, 5-LO activity is important for immune defense, whereas improper regulation contributes to pathogenesis, including chronic inflammation, asthma and atherosclerosis. Like all lipoxygenases, the 5-LO protein consists of two domains, a regulatory domain and a catalytic domain. Naturally, the regulatory domain determines catalytic activity and controls leukotriene synthesis. This domain shares features with classical C2 domains in that it has a β-sandwich structure and binds calcium, nucleotides and phospholipids. However, important structural features place this domain in a distinct family, the PLATs (for Polycystin-1, Lipoxygenase, α-Toxin). In this review, we summarize our current understanding of the three dimensional organization of this important component of the 5-LO molecule. In addition, we point to findings from structural analyses of related proteins to suggest further details relating 5-LO structure to function.