After a brief introductory and historical overview on electrophysiology (Chapter II:Introduction) basic electrochemical principles for the understanding of this subject are summarized (Chapter III: Basics: Theory). Thereafter, electrophysiological methods including principles of data analysis are presented in the 4th chapter (IV: Basics: Methods) ranging from measurements on the whole animal via measurements on singe cells with microelectrodes to the patch-clamp technique. This chapter also includes a brief presentation of ion-selective microelectrodes and the carbon-fibre technique. In the following chapter (V: Ion-selective Channels), the major conductance pathways in cell membranes are described with respect to their specific characteristics. Essentials of membrane excitability based on Hodgkin-Huxley decription of an action potential and synaptic transmission are presented in the 6th chapter (VI: Theory of Excitability). This is followed by a chapter (VII: Carrier-mediated Transport) presenting with three examples characteristics of carriers compared to channels and how electrophysiological methods can be used for functional characterisation. Finally, we finish with exemplary illustration of how combination of electrophysiology, molecular biology and pharmacology can be applied to learn about structure, function and regulation of membrane permeabilities that form the basis of cellular function (Chapter VIII: Modern Examples of Application of Electrophysiology).