The paper describes the physical phenomena involved in the conduction of electricity, with particular reference to living tissue. The conduction of electricity depends on the flow of charge carriers in the material, while the dielectric properties are due to the rotation of dipoles that can align along an applied electric field. The relation between the electric variables in a conducting medium and their physical meanings are explained. The phenomena responsible for the electric and dielectric properties of living tissues are described. The presence of cells limits the flow of charge carriers, in particular at low frequency, and the membranes are responsible for dielectric relaxation. The passive response of cell membranes to weak applied signals enables bioelectrical tissue characterisation. Practical tools for electrical impedance spectroscopy are given with an overview of the most recent applications.