The stellarator is a 3-dimensional toroidal configuration without induced toroidal plasma currents. The magnetic field is produced by external coils which allows steady-state operation of a stellarator reactor. The requirements coming from reactor conditions narrow the range of potential stellarator configurations as reactor candidates; therefore an optimization of stellarator configurations is necessary to meet these requirements. The Helias concept developed at the IPP Garching offers the chance of stable plasma confinement up to beta =5% and predicts neoclassical transport losses below the critical level for ignition. This can be achieved by reducing the radial drift of trapped and circulating particles as far as possible (drift-optimization). The neoclassical transport of trapped particles can be characterized by an effective helical ripple of 1-2%. The theory of an optimized stellarator reactor mainly concentrates on the issues of plasma equilibrium, MHD-stability, neoclassical effects and fast particle behavior. Much effort is also devoted to the optimization of the coil system as the main technical component of the reactor. The dimensions of a hypothetical Helias reactor are: major radius 20 m, average plasma radius 1.6 m, magnetic field on axis 5 Tesla, rotational transform in the range 5/6<i<1. Details of this reactor concept are described elsewhere (Beidler et al. 1992).