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A general description of arc cathode spots is given with respect to structure, size, and temporal behavior. The density of the metal vapor plasma outside the spot is presented as a function of the coordinates, calculated on the basis of ion currents flowing to screens and probes. Extrapolation to the region within the spot indicates plasma densities in excess of 1025 m-3. Recent studies of spot current density suggest values of about 1012 A/m2. In this connection, the problem of the electrical conductivity of the spot plasma is discussed. Finally, experiments on spot lifetime and models of crater formation are reviewed.