The document discusses the evolution of resource discovery tools in libraries from online catalogs to current discovery services. It outlines the components of discovery interfaces, including the user interface, local search capabilities, and ability to search remote indexes. Examples of popular discovery interface products are provided. The trends in discovery include a shift from locally-installed to hosted, pre-populated indexes aiming for web-scale discovery. The future of discovery is envisioned to include more comprehensive indexes, stronger search technologies, discovery beyond library interfaces, and linked data.