The several independent evolutions of endothermy in the history of vertebrates are key innovations because these changes modified the energetic relationships between organisms and their environments. These adaptations were correlated with major changes in the respiratory and circulatory systems. Bone histology has also been used by some authors as a proxy for energetics and thermometabolic regime (e.g., endothermy vs ectothermy) in extinct vertebrates. This chapter contains an historical overview of paleophysiology, the discipline in which these inferences are performed, and two reviews. The first section deals with the biochemical mechanisms involved in thermogenesis, a necessary basis for rigorous discussions about paleobiological inferences. The second section is an overview of thermometabolic analyses performed on different vertebrate groups, especially synapsids and diapsids, using bone histology.