Index construction interacts with several topics covered in other chapters. The indexer needs raw text, but documents are encoded in many ways (see Chapter 2). Indexers compress and decompress intermediate files and the final index (see Chapter 5). In web search, documents are not on a local file system, but have to be spidered or crawled (see Chapter 20). In enter- prise search, most documents are encapsulated in varied content manage- ment systems, email applications, and databases. We give some examples in Section 4.7. Although most of these applications can be accessed via http, native ApplicationProgramming Interfaces(APIs)areusually more efficient. The reader should be aware that building the subsystem that feeds raw text to the indexing process can in itself be a challenging problem.