A brief historical account is given of the discovery of OH emission and the experiments that led to the identification of the radiation mechanism as maser amplification. An outline is presented of the progress by the MIT, CfA and Jodrell Bank groups in mapping the prototypical maser, W3(OH). The measurements of the structure and proper motions of masers are described and their limitations explained. Studies of the structure of extragalactic water vapor masers are summarized.