Web search has significantly evolved in recent years. For many years, web search engines such as Google and Yahoo! were only providing search service over text documents. Aggregated search was one of the first steps to go beyond text search, and was the beginning of a new era for information seeking and retrieval. These days, web search engines support aggregated search over a number of verticals, and blend different types of documents (e.g. images, videos) in their search results. Moreover, web search engines have started to crawl and search the hidden web. Federated search (federated information retrieval or distributed information retrieval) has played a key role in providing the technology for aggregated search and crawling the hidden web. The application of federated search is not limited to the web search engines. There are many scenarios such as digital libraries in which information is distributed across different sources/servers. Peer-to-peer networks and personalized search are two examples in which federated search has been successfully used for searching multiple independent collections. Federated Search provides a comprehensive summary of the research done to date, looks at some of the challenges still to be faced, and suggests some directions for future research on this important and current topic.