The definition and basic equations defining the performance of three-dimensional radar systems are given. In particular the parameters defining data rate for different classes of three-dimensional radar are analyzed. Three-dimensional radars are divided into three classes as follows: single beam-rapid scan systems; multiple beam-scanning systems; and multiple beam-non-scanning systems. Each of these types is defined and compared in terms of important characteristics. It is shown that new developments in electronic scanning and multiple beam antennas have made feasible many of these new three-dimensional radar systems. A number of the more important antennas and their method of utilization in the three-dimensional radar equipment are described. Examples are given of two types of three-dimensional radar that are typical of two classes of this equipment. Finally, some of the more important applications for modern three-dimensional radar equipment are noted.