Taking as the most recent example the vacuum system of the large electron and positron (LEP) storage ring at CERN, general design considerations for accelerator vacuum systems are discussed. The vacuum system for particle accelerators must provide a low residual gas pressure to meet the basic requirement of a long beam lifetime. During operation, the pressure is strongly affected by direct or indirect interactions of the particle beam with the surrounding vacuum envelope and reveals a beam dependent ‘‘dynamic’’ behavior. The various processes of gas desorption call for specific conditioning methods of the vacuum vessel—e.g., conventional baking complemented withinsituconditioning using the beam—for which the provisions must be included during the design and construction phase. The mechanical choices for the vacuum system have to satisfy external constraints and are intimately linked to the major accelerator systems like magnet design and accelerating sections. Modern accelerators have grown to very large linear size, which leads to the practical limitation that their vacuum systems are strongly dominated by the limited conductance of the vacuum duct—a problem that has been overcome by the extensive use of ‘‘distributed’’ or ‘‘linear integrated’’ pumps. Because of the size and the large number of components, low cost per unit length and high reliability are of prime importance.