For a fusion power plant to be economically and technologically attractive, it should
be as compact as possible and capable of `steady state' operation. One approach is based upon
the spherical tokamak (ST) concept. This configuration features many of the qualities of the
conventional aspect ratio tokamak, such as good confinement and MHD stability, together with a
number of highly promising features for the realization of a cost effective, steady state
fusion power core. In particular, it allows high β capability due to the high I/B
achievable at low A and strong, natural shaping of the configuration, together with the
possibility of approaching 100% pressure driven currents through high natural elongation
and access to second stability to ideal ballooning modes. The physics insights gained from
experiments on START are discussed and the theoretical modelling work summarized, describing
how the beneficial properties of STs can be combined in steady state ST power plant designs
such as the Culham STPP concept. Where appropriate, we shall refer to the two new mega-amp
machines MAST and NSTX, both designed for a plasma duration approaching the plasma current
relaxation time and also other new, smaller, STs, designed to expand the knowledge gained
so far.