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Hibernation in the Ursidae has been extensively researched over the past 30 years. This paper reviews findings of that research in the areas of general physiology and energetics; protein, fat, and bone metabolism; metabolic endocrinology; reproductive physiology and lactation; serum chemistry and hematology; and the urea:creatinine ratio. Bears in hibernation exhibit several characteristics distinct from the deep hibernation of rodents, such as a lesser reduction in body temperature, protein conservation, lack of defecation and urination, and normal bone activity. The physiological constraints of hibernation are coupled to adaptations in reproductive physiology, such as delayed implantation and lactation. I argue that urea:creatinine is not a reliable indicator of hibernation, although ongoing research is searching for an opioid-like hibernation trigger. Study of hibernation physiology will continue to bear fruit, especially in the areas of evolution, physiology, and medicine.