Eicosa-, Greek for “twenty,” are signaling molecules derived by oxygenation from omega-3 (ω-3) or omega-6 (ω-6) fatty acids. They exert complex control over many bodily systems, especially in inflammation, immunity, and as messengers in the central nervous system. The networks of controls that depend upon eicosanoids are among the most complex in the human body. There are four families of eicosanoids – the prostaglandins, the prostacyclins, the thromboxanes, and the leukotrienes. For each, there are two or three separate series, derived either from an ω-3 or ω-6 essential fatty acid.