Sequential injection analysis (SIA) a third generation of flow injection analysis (FIA), shows a new style of flow injection system The new system, utilizing a modified sample introduction unit in conjunction with a compact flow design, fulfills the requirements of minimum maintenance, simple and user friendly flow schemes. The SIA is suitable for various kinds of analysis, In comparison to FlA. it is more flexible for Slop-flow and reversed flow operations. More than thirty publications dealing with the principle and application of SIA were published since its first launch in 1990. This paper discusses various aspects of the SIA, including the rationale behind the development. principle, advantage s, disadvantages and applications. Furthermore, a SIA system coupling with an immobihzed enzyme reactor was used as an example 10 highlight the future development of SIA.