A study has been made to determine the basic phenomena associated with the ground interference effects for VTOL aircraft in an attempt to arrive at some generalized conclusions regarding the effect of the ground on the various types of VTOL aircraft. The results showed that helicopter and other rotor aircraft generally experience favorable ground effect. In the case of propeller VTOL aircraft the tilt-wing configurations will usually experience a small favorable ground effect. For the deflected-slipstream configurations a large detrimental ground effect is experienced at low angles of the thrust axis but little or no ground effect is experienced at angles of 25 degrees or 30 degrees. For buried-fan and jet configurations the ground effect can be favorable or unfavorable depending upon the geometry of the fans or jets and the airframe. For single-jet configurations the ground effects are detrimental and cannot be eliminated by fixes on the airplane. For these cases, a perforated landing platform appears to offer promise as a means of minimizing the adverse ground effect. Ground effects experienced in hovering flight tend to decrease with increasing forward speed and are rather small at airspeeds that might be of interest for STOL operation of the various VTOL configurations. The qualitative predictions of the ground effect for various VTOL aircraft configurations can be made with a fair degree of confidence, but it appears at the present time that the magnitude of the effect for specific configurations will usually have to be obtained from test data.