Uncoupling proteins (UCPs) are regulated H+ transporters and a subfamily of the mitochondrial carrier family. Whereas UCP1 in brown adipose tissue has a well-defined role in thermogenesis, the roles of other UCPs are still tentative, such as in control of immune response, oxygen radical formation, and insulin secretion. The popular overexpression in yeast did not yield a functional form of UCP3 and possibly of other UCPs in mitochondria with the exception of UCP1. Whereas UCP1 can be isolated in native form, the isolation of other native UCPs from tissues or from overexpression in yeast failed. UCPs (UCP1, 2, and 3) expressed in E. coli as inclusion bodies can be reconstituted to yield H+ transport only in the presence of CoQ requiring fatty acids as native UCP1. The rates are similar to native UCP1 and are inhibited by low nucleotide concentrations. Native UCP1 is activated by endogenous CoQ. Differences between UCPs may reside in the regulation, such as by the ATP/ADP ratio in accordance with the specific cellular requirements.