The atomic hydrogen maser is described. In this device hydrogen atoms in the upper hyperfine state are focused onto the entrance aperture of a teflon coated quartz bulb in which the atoms are stored for about a second. This bulb is surrounded by a cylindrical radiofrequency cavity. When the cavity is tuned to the hyperfine frequency of atomic hydrogen, maser radiation is produced. Due to the large line Q resulting from the long storage time, the radiation is highly stable in frequency. Results are given of theoretical calculations on the threshold flux of atoms required for maser oscillations, on the various relaxation processes that limit the effective storage time, and on the possible sources of frequency shifts of the maser. Results are given on the relative stability of two hydrogen masers. Measurements of the atomic hyperfine frequency of atomic hydrogen and deuterium give Δ_H = 1,420,405,751.800 ± 0.028 cps and Δ_D = 327,384,352.5 ± 1.0 cps on the A.1 time scale with Δ_Cs = 9,192,631,770.0 cps. The method by which the deuterium has been measured depends upon the effect of a deuterium magnetic resonance transition upon the intensity of the hydrogen maser oscillation amplitude when a mixture of hydrogen and deuterium is used. This method should be capable of extension to a number of different atoms. The hydrogen maser apparatus has been used to measure the spin exchange collision cross section between atomic hydrogen and a number of different gases. Results of some of these measurements are reported. Measurements of the dependence of the hydrogen hyperfine frequency upon a strong externally applied electrostatic field are given.