The effects of operation near the ground are of primary importance in determining the takeoff and landing characteristics of STOL aircraft. A study is presented which compares the results of theoretical analysis with those of wind-tunnel tests using a fixed ground plane with blowing boundary-layer control as well as the moving belt ground plane. These results are used to define the characteristics of STOL lifting systems near the ground to serve as a basis for an examination of limitations imposed on STOL operation. Variations in aircraft design parameters and operating techniques are explored to indicate ways of reducing the adverse effects of ground proximity.