Long-chain-fatty-aid CoA ligases (EC 126.96.36.199) catalyse the bioactivation of fatty acids forming acyl-CoA thioesters that are then substrates for anabolic and catabolic pathways. In addition to these roles it is now recognised that fatty acyl-CoA esters are key regulatory molecules affecting numerous cellular systems and processes such as cell signalling, membrane fusion, protein acylation, protein kinase C activity, and gene transcription as natural ligands of the peroxisome proliferator-activated receptors. The key to both fatty acid activation and xenobiotic acyl-CoA formation is the role played by the long-chain ligases (LCL) that exist as a super family of membrane proteins. Focussing on information relevant to humans, multiplicity of LCLs, their structural features, and regulation are discussed. The fate of fatty acyl-CoAs and the role of LCL in directing fatty acyl-CoA traffic are also considered as these are integral to an appreciation of the consequences of xenobiotic-CoA conjugation and the formation of lipophilic conjugates. Although fatty acid activation is considered crucial to the provision of bioactive regulatory molecules, xenobiotic-CoA conjugation is at times a barely recognised route of drug metabolism. Knowledge of the consequences of xenobiotic-CoA formation and the pervasive intracellular role these conjugates play can provide further insight into xenobiotic metabolism and the interrelationship with lipid metabolism.