This book covers the molecular structures and the cellular and in vivo function of endosomes and lysosomes, i.e. intracellular vesicles which are involved in many cellular processes such as endocytosis, intracellular trafficking, degradation of material from inside (e.g. autophagy) and outside the cell as well as exocytosis. Membranes of endolysosomal organelles contain an amazing number and diversity of ion channels. These ion channels are the topic of the present book that focusses on describing the structure, the biophysical properties, physiological functions of endolysosomal ion channels at the molecular, cellular and in vivo level.