Antenna arrays exist in a wide variety of configurations, with the number of radiating elements ranging from two to several thousand. However, all are governed by the same fundamental principles, which are described in this article Contemporary antenna problems have stimulated renewed interest in the use of array-type antennas, in which a number of individual radiators are grouped together and coherently excited in some controlled manner. This type of antenna predates the application of reflector types in radio and radar applications; but the new interest, with emphasis on arrays of large numbers of elements, has provided the impetus for advances in our knowledge of the principles of arrays and the invention of new arraying techniques. It is the aim of this article to provide an exposition of the current state of the field.