Web scale discovery services are a tool with major potential to transform the nature of library systems. These services are capable of searching quickly and seamlessly across a vast range of local and remote content and providing relevancy-ranked results in the type of intuitive interface that today s information seekers expect. This report describes in detail the content, interface, and functionality of web scale discovery services developed by four major library vendors: OCLC, Serials Solutions, Ebsco, and Ex Libris. Each of these services is evolving rapidly, indicative of their open framework design and an ongoing expansion of indexed content as additional publisher and aggregator agreements are brokered. Although many similarities among the services are apparent, this report also outlines some observed differences, though these differences are becoming hazy as each vendor adds new functions, features, and content. To help individual libraries evaluate which service will best meet the needs of the library and its community, this report provides detailed evaluation questions and concludes with a section providing additional background information on each service.