Astrophysical masers, naturally occurring microwave and radio-wave emissions, provide an important tool to investigate astrophysical environments. Several thousand maser sources have been observed in the interstellar medium and star formation regions, in the expanding winds from evolved stars, in the compressed shells of supernovae remnants, in comets and in the centers of galaxies. Masers are also useful for studying the structure and dynamics of our own galaxy, while masers in other galaxies are now used for cosmological studies and studies of galactic nuclei. This volume contains the latest research on the topic as presented at IAU Symposium 242, only the third international symposium on astrophysical masers. Over 125 astronomers from 18 different countries gathered in Alice Spring, Australia, for discussions on maser research.