This chapter discusses the properties and physiological significance of fatty acid binding properties. Fatty acid binding proteins (FABP) are found in many tissues of many different organisms, which include mammals, fish, birds, and insects. All FABPs are members of a large multigene family called intracellular lipid binding proteins (iLBPs) with various functions in the transport and metabolism of their ligand fatty acids and other lipophilic ligands. There are different roles in different cells, tissues, and organisms may vary, common features become apparent in the context of metabolic tasks and conditions. The purpose of this chapter is to summarize current knowledge about these proteins, and to provide insight into their roles in different organisms. FABPs are expressed in vertebrate and invertebrate species. Pertaining to the latter, two FABPs are expressed in the midgut of the tobacco hornworm and believed to be involved in lipid digestion. The FABP from the flight muscle of locusts is especially well characterized. The chapter also explains structure and conformation of FABPs and their ligands.