The resonant cavities are structures used to store the electromagnetic energy at high frequencies. Cavities may be rectangular, cylindrical, or spherical in geometry. This chapter is devoted to discuss the rectangular and cylindrical cavities and their characteristics. The electromagnetic fields in the rectangular and cylindrical cavities are obtained by considering the cavities as shorted waveguides at their ends with two additional walls. The most important parameters that characterize the cavity are its resonant frequency and quality factor. The quality factor is a parameter that gives a measure of the ratio of the stored energy in the cavity to the dissipated power at a certain frequency. A detailed derivation for the electromagnetic fields, resonant frequency, and the quality factor are presented for both the rectangular and cylindrical cavities. The topics of the chapter are supported by a number of illustrative examples, figures, and solved problems. Homework problems are included at the end of the chapter.