Design specifications for plasma immersion ion implantation systems are discussed. An analytical model of the plasma sheath evolution is used to deduce restrictions for the dimension of the vacuum chamber and the pressure range. The avoidance of arcing implies an upper limit for the maximum electric field in the sheath and hence for the plasma density. The total fluence per pulse, which determines the implantation duration and the power requirement, is calculated. Furthermore, safety considerations, especially the shielding of x-rays generated by secondary electrons, are discussed.