Between them, bioacoustic libraries worldwide preserve samples of sounds of most of the vertebrate species that utilise acoustic communication, and many of the most important invertebrates. Their collections have been built up mainly through the contributions of many scientists and recordists and represent many hundreds of thousands of hours of work in the field. They are invaluable especially for comparative studies between individuals, populations and species where it is often impossible for one person to replicate, even in a lifetime of work, the dedicated efforts of so many collectors of sounds. Examples of the use of sound libraries in biodiversity studies include: providing identification tools in fieldwork; documenting biodiversity by sound sampling; analyses of community sound structure and the "acoustic niche'' hypothesis; and as clues for taxonomic and phylogenetic analyses. Sound libraries are gradually improving facilities towards providing remote access to their collections. Preliminary catalogues of two large collections - those of the National Sound Archive in London and of the Borror collection in Ohio - are already accessible on the Internet and a small but growing number of Internet sites offer instant access to actual audio samples.