In a multiuser communication system, a multiple access scheme is required to share a common channel resource, e.g. the frequency band. For mobile multiuser communication systems, especially cellular systems, a code division multiple access (CDMA) scheme is widely employed due to several advantages over other schemes. In CDMA systems, users can transmit signals simultaneously over the same frequency band. This makes CDMA systems interference-limited since interfering signals (i.e. other users signals) degrade the performance. To improve the performance of CDMA systems, multiuser detection has been investigated. Multiuser detection attempts to detect the desired signal as well as other signals (this implies a joint detection ^{[ ]}) for a better performance. Indeed, this is an analogy of the MLSD, in which a symbol sequence (i.e. multiple symbols), rather than individual symbols, is detected. There are also a number of different approaches taken to mitigate or suppress interfering signals for multiuser detection. In this chapter, we introduce CDMA systems and multiuser detectors; adaptive multiuser detectors are also introduced.