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The large size and the dynamic nature of the Web make it necessary to continually maintain Web based information retrieval systems. Crawlers facilitate this process by following hyperlinks in Web pages to automatically download new and updated Web pages. While some systems rely on crawlers that exhaustively crawl the Web, others incorporate "focus" within their crawlers to harvest application-or topic-specific collections. In this chapter we discuss the basic issues related to developing andoi:10.1007/978-3-662-10874-1_7 fatcat:wz2wsoi3d5h2vebem2rof2jv2e