Electrorotation is the circular movement of an electrically polarized particle in a rotating electric field. Similar to the slip of an electric motor, it arises from a phase lag between the applied rotating electric field and the ensuing relaxation processes within the particle. Electrorotation may thus be used to investigate these relaxation processes or, if these are known or can be accurately modeled, to determine particle properties. The method is frequently employed as a non-invasive means of characterizing biological membranes, e.g. in terms of their membrane thickness or electrical conductivity in cellular compartments.