The tutorial provides current insights into the electrophysiology and molecular biology of cell membranes. The issues of molecular organization of biological membranes, passive electrical properties of membranes, the path of movement of ions through cell membranes are covered. General ideas on structure and functions of ion channels are given. Passive ion transport is described, cell potentials and their relationship to ion currents are considered. Molecular organization and functions of voltage-gated sodium, calcium and potassium channels are presented, certain aspects of ligand-gated channels are discussed, operation of mechanosensitive channels is considered in detail. New classification of channels is given.