<div class="htmlview paragraph">A careful analysis is made of the basic principles which govern the operation of those ground-cushion devices currently known as ground-effect machines (GEM’s).</div> <div class="htmlview paragraph">Most GEM’s can be defined as machines which operate in close proximity to the earth’s surface without ever physically touching it because they are always separated from it by a cushion or a layer of air, however thin; as a consequence, propulsion and control of GEM’s must be of an aerodynamic nature. The only machines not covered by this definition are special types of over-water GEM’s.</div> <div class="htmlview paragraph">It is proposed to establish six main categories of GEM’s from which all other possible configurations could be evolved by proper combination of the basic types. For each of these six configurations, a critical bibliographical survey is made and some comparative features are pointed out. The overall picture is one of great complexity because of the large number of parameters involved. This points out the danger of a “hardware” approach to the GEM problem until a firm base of comprehensive research has been established. At the present time, the emphasis placed on the annular jet seems justified, inasmuch as the annular jet represents a good compromise between versatility, simplicity and performance.</div>