Interference effects between a highly underexpanded, sonic or supersonic jet in a subsonic or supersonic crossflow, and the surface from which the jet exhausts are examined. For subsonic freestream Mach numbers, existing data are examined and correlated. Various semi-empirical models to represent the interference pressure distribution on flat plates are then developed. For supersonic freestream Mach numbers, a computer program for calculating jet interference effects on axisymmetric bodies at angle of attack is described. Interference effects between the jet plume and control fins on a cruciform missile are analyzed. A semi-empirical model of the jet in a crossflow, valid at large distances from the nozzle is developed. The results of this model are then used to compute interference forces and moments on fins located aft of the nozzle, both for subsonic and for supersonic freestream Mach numbers.