Since at least the time of the Romans, people have known organizations by shortened names -- acronyms, initialisms, and abbreviations. While motivations for doing so have varied, they have generally included economy of space and ease of communication. In many cases, the abbreviation is more widely recognized than the formal organizational name. This book collects several thousand acronyms, initialisms, and abbreviations for organizations, mostly in use in the past two or three centuries and largely within the United States, and provides the researcher with possible meanings for what may, in the beginning, appear to be an indecipherable string of letters that contextually appear to belong to an organization of some type. Also provided for each entry is an attempt at categorizing the organization, since many of these strings of letters potentially refer to a number of different organizations, often of entirely different types. For academic institutions, an indication of location is also generally provided.