One of the most spectacular discoveries of molecular astronomy has been the detection of maser emission. The same radiation that is generated in the laboratory only with elaborate, special equipment occurs naturally in interstellar space. This intense radiation probes the smallest structures that can be studied with radio telescopes. By a fortunate coincidence maser radiation is generated in both star forming regions and the enveloped of late-type stars. The early and late stages in the life of a star are considered to be the most interesting phases of stellar evolution. Maser emission has also been detected in external galaxies. This book provides an extensive coverage of the interstellar maser phenomenon. A precondition for maser action is departure from thermal equilibrium. The book therefore starts with a detailed coverage of the basic background concepts required for an understanding of line formation and radiative transfer. It goes on to describe the theoretical and phenomenological aspects of interstellar masers, their formation sites and the inversion mechanisms. The book should interest active researchers in astronomy and astrophysics as well as other areas of physics. It is designed to be suitable as a textbook in a graduate course and aims to enable a graduate student to embark on research projects in this area in particular, and molecular radio astronomy in general. Basic background concepts; astronomical maser radiation; maser theory; effects of geometry; polarization; pumping; the environments of astronomical masers; OH masers; H2O masers; SiO masers; other masers; extragalactic masers; masers as astronomical tools.