This review is concerned with the physics of field-reversed configurations created by energetic particles of large orbits, of which the earliest example is the Astron. The Astron concept has evolved into the Ion Ring Compressor in which a low-energy ring of ions is magnetically compressed to high energy for heating the confined plasma to thermonuclear ignition. A third version is the hybrid in which field reversal is created both by plasma currents as in a θ-pinch or a spheromak and a component of energetic large orbit ions to provide heating energy as well as improved stability. This paper treats first the injection, trapping, and formation of electron and ion rings from a theoretical standpoint as well as presenting the principal experimental results. The equilibria and the characteristics of the particle orbits are discussed in some detail including conditions under which the orbits cease to be integrable and pass to a stochastic description. The theory of magnetic compression of rings and the classical transport of heat and particles of the plasma confined in the closed-poloidal-field-line region is presented. The bulk of the review is concerned with low-frequency stability of such configurations by means of a generalized energy principle for treating the large-orbit particles in the Vlasov limit. The pathology of all possible unstable modes is discussed at some length. We conclude with an exhaustive survey of microinstabilities driven by the energetic particle current for a special model and conclude that most of them are stabilized by strong gradients in density and magnetic field. A brief discussion on drift modes follows. Our present theoretical understanding of these systems, although still incomplete in some respects, nevertheless leads to the view that the promise of the very favourable features of fusion reactors based on these concepts is a sufficient impetus for their continued experimental and theoretical study.