The versatility of the JET device allows a wide range of tokamak operating regimes to be explored and plasmas bounded both by material limiters and by a magnetic separatrix have been investigated extensively. This has permitted the confinement and MHD stability properties of plasmas heated to temperatures above 10 keV by neutral beam injections or ion cyclotron resonance heating to be studied in detail. The results of analyses of transport and confinement in the L- and H-mode regimes in JET are discussed and the properties of H-mode plasmas produced by both major forms of heating are compared. Several aspects of the MHD stability of such plasmas, particularly at high toroidal beta beta _psi, and at the density limit, are reviewed.