The paper describes the basic mechanisms of Hot Gas Recirculation (HGR) in V/STOL aircraft, and some ways to assess and control it. Severe HGR can cause large thrust losses, or even compressor stall; avoidance of excessive HGR is therefore critical for safe V/STOL operation, especially when a high-specific-thrust engine is used to give the aircraft supersonic capability.Near the ground, jet-induced lift losses increase, but can be offset by upward forces resulting from “fountain” flows. The fountain is a powerful potential HGR source; the ground jet can also be a source of HGR through its interaction with headwind, aircraft motion and buoyancy. Some practical design principles to limit the resulting HGR are discussed.The full scale data base on HGR is limited, and it is therefore necessary to rely heavily on model testing. Scaling principles are briefly surveyed; it is shown that it is not possible to achieve similarity in all test parameters at once. Likely ways out of this dilemma are discussed with reference to some existing test facilities. Computational fluid dynamic (CFD) simulations are showing promise, but a full time-dependent HGR computation is still beyond the capability of generally available computers. Improvement in CFD methods is likely to be paced by our knowledge of the underlying mechanisms of entrainment and heat exchange.