Part I: Physiological Functions of Endo-lysosomal Cation Channels -- NAADP-Mediated Ca2+ Signalling -- NAADP-Dependent TPC Current -- NAADP-Evoked Ca2+ Signaling: The DUOX2-HN1L/JPT2-Ryanodine Receptor 1 Axis -- TPC Functions in the Immune System -- Lysosomal Ion Channels and Lysosome- 1 Organelle Interactions -- TRPML1 and TFEB, an Intimate Affair -- Lysosomal Potassium Channels -- Part II Structure and Composition of TPC and TRPML Channels -- Structure and Function of Plant and Mammalian TPC Channels -- A Structural Overview of TRPML1 and the TRPML Family -- Endo-Lysosomal Two-Pore Channels and Their Protein Partners -- Part III: Tools and Methods to Characterize Endolysosomal Cation Channels -- Electrophysiological Techniques on the Study of Endolysosomal Ion Channels -- The Plant Vacuole as Heterologous System to Characterize the Functional Properties of TPC Channels -- Expanding the Toolbox: Novel Modulators of Endolysosomal Cation Channels -- Characterization of Endo-Lysosomal Cation Channels Using Calcium Imaging.