The role of free fatty acids (FFAs) as a source of energy and their functions in energy transport within the body are well established. Equally important is a role that FFAs play in oxidative stress following cell membrane depolarization. FFAs are physiologically active, not only as nutritional components, but also as molecules involved in cell signaling and stabilization of membranes via palmitoylation and myristoylation. Protein palmitoylation is involved in numerous cellular processes, including apoptosis, and neuronal transmission. Besides nuclear peroxisome proliferator-activated receptors that mediate the biological effects of FFAs, G protein-coupled receptors (GPCRs) that are activated by FFA, have been recently identified. Those multiple FFA receptors (FFARs), which function on the cell surface as activated FFAs, play significant roles in the regulation of energy metabolism and mediate a wide range of important metabolic processes. FFARs have been targeted in drug development for the treatment of type 2 diabetes and metabolic syndrome. FFAs upregulate transcription of uncoupling proteins, increasing their expression in brain, cardiac, and skeletal muscle that may be protective or cytotoxic, depending on the cellular energy state. Recently, FFA effects on the endothelial function and dysfunction are being recognized. FFAs play a key role in endothelium-dependent nitric oxide production. A disturbance of endothelial function, due to an imbalance in production and release of relaxing and constricting factors, has implications in the development of cardiovascular problems, such as hypertension, as well as neurotoxicity following loss of blood-brain barrier integrity. This review presents information on broad range of FFAs actions of prime importance for physiological processes. Understanding of FFA functions in the body is crucial for developing new therapeutic strategies against several metabolic disorders.