The traditional picture of an endotherm is of an organism that maintains a high constant body temperature, fuelled by a correspondingly high food intake.  Recent studies using a suite of new logging devices have shown that the reality is a lot more nuanced.  Birds and mammals allow their body temperatures to vary on a daily and seasonal basis, have different set-points for pregnant females compared with males, and also allow different parts of the body to have different temperatures in response to environmental challenges.  In this talk I will summarise recent work, and discuss the light it throws on the evolution of endothermy, torpor and hibernation.  This shift in our view also has important consequences for understanding how endotherms may respond to continuing climate change.