Autophagy is an evolutionarily conserved cellular catabolic mechanism for the turnover of organelles and proteins, through a lysosome-dependent degradation pathway. Autophagy is an essential function for cell homeostasis and adaptation to environmental stress conditions including nutritional starvation, energy depletion, endoplasmic reticulum stress, oxidative stress, and hypoxia. The role of autophagy in diseases is being studied, with indications that autophagy may be both adaptive and antagonistic depending on context, where deficiency or absence in autophagic function may play a pathogenic role in select human diseases.