An antenna is an element carrying a time-varying current that radiates an electromagnetic wave. Such an element is designed to achieve certain properties. Antenna properties can be classified into three categories, as proposed by Rothwell and Cloud [1]. These are (i) radiation properties, (ii) circuit properties, and (iii) radiation-circuit properties. After defining antenna regions in Sect. 12.1, those properties are discussed, respectively, in Sects. 12.2, 12.3, and 12.4. A useful computer program is given in the appendix at the end of the chapter.