The operation to separate dust from a fluid stream is referred to as particle collection. It has been utilized to recover valuable materials from many chemical processes and to remove air pollutants consisting of particulate matter from exhaust streams before exiting to the atmosphere. In this operation, it is essential to produce a suficiently large relative velocity between the particles and the fluid. Hence, many types of collectors have been developed and used to accomplish the tasks mentioned. In this chapter, the principles and features of particle separation and typical particle collectors are described. Particle separation in a hydro-cyclone is also described in Section 15.13, because the separation process of particle is similar in both liquid and air.