In this paper a general formula is derived for the power radiated in nondissipative media by a given distribution of electric and magnetic currents. Magnetic currents are included not only for the sake of greater generality but also because in problems involving diffraction through apertures and radiation from electric horns, the radiation intensity can be made to depend upon fictitious electric- and magnetic-current sheets covering the apertures or horn openings. Part I consists of an introductory discussion, summary of the formulas, and examples illustrating the convenience of the general formulas. Part II contains a mathematical derivation of the radiation formulas.