Bioimpedance describes the passive electrical properties of biological materials and serves as an indirect transducing mechanism for physiological events, often in cases where no specific transducer for that event exists.

The studied biomaterial may be living tissue, dead tissue, or organic material. Tissue is composed of cells with poorly conducting, thin-cell membranes; therefore, tissue has capacitive properties: the higher the frequency, the lower the impedance. Tissue is also anisotropic. Impedance spectroscopy, hence, gives important information about tissue and membrane structures, as well as intra- and extracellular liquid distributions.

The three most common electrode systems are described. Typical results obtained with three- and four-electrode systems are shown, and dispersion is explained. The effect of electrode polarization is demonstrated.

The importance of system geometry, sensitivity, and reciprocity is explained. How bioimpedance can be used for volume measurements (plethysmography) is described. The sensitivity field of an electrode system is defined and explained and examples are provided.

Bioimpedance electrical models are based on tissue showing relaxation, but not resonance, phenomena. The choice between an impedance or admittance model is underlined. The Constant Phase Element (CPE) and the Cole models are critically described, and a new general model is introduced.

Some electrode design concepts are treated. Lock-in amplifiers are the preferred instrumentation for bioimpedance measurements, together with advanced software for data analysis. The electrical safety aspects are treated.

Selected applications and future trends are described. Bibliography and sources for reference data are given.