[en] Collective phenomena in circular accelerators and storage rings, caused by the presence of a large number of particles in the beam are discussed. A large number of particles moving in their surroundings create sizable fields which act back on the beam. The most important collective phenomena are instabilities; a small deviation from the nominal distribution of particles in longitudinal position and energy creates fields which increase this initial deviation. Such longitudinal instabilities are investigated in three steps. Firstly the properties of the guide field which determines the longitudinal behaviour of a single particle of given momentum, and the nominal distribution in longitudinal position and energy of the particles have to be known. Secondly the fields and induced wall currents due to a small deviation from the nominal distribution are calculated, the forces acting back on the beam (self forces) are determined. Thirdly the change in the particle distribution due to these self forces is calculated and it is observed whether the initial deviation is increased (instability) or decreased (stability). This paper is restricted to phenomena which are important in circular accelerators and storage rings under normal operating conditions, neglecting e.g. effects at transition energy etc. Bunched and unbunched (coasting beams) are discussed separately. (Auth.)