Lysophosphatidic acid (LPA) is one of the simplest glycerophospholipids with one fatty acid chain and a phosphate group as a polar head. Although LPA had been viewed just as a metabolic intermediate in de novo lipid synthetic pathways, it has recently been paid much attention as a lipid mediator. LPA exerts many kinds of cellular processes, such as cell proliferation and smooth muscle contraction, through cognate G protein-coupled receptors. Because lipids are not coded by the genome directly, it is difficult to know their patho- and physiological roles. However, recent studies have identified several key factors mediating the biological roles of LPA, such as receptors and producing enzymes. In addition, studies of transgenic and gene knockout animals for these LPA-related genes, have revealed the biological significance of LPA. In this review we will summarize recent advances in the studies of LPA production and its roles in both physiological and pathological conditions.