In powder handling, each particle collides with another particle or a wall, and consequently becomes charged up to a certain value. Such contact charging is experienced in various fields. In the present review, the basic concepts of contact charging are summarized; in particular, the effect of the contact potential difference and the initial charge on the charge transfer is described in detail. Furthermore, the variation of the particle charging caused by repeated impacts on a wall is formulated. This theory is extended to the particle charging in gas-solids pipe flow, where each particle has a different amount of charge; the distribution of the particle charge is also analyzed theoretically. In addition, the method of measuring important electrostatic properties, the technique of detecting particle charging and the application of particle charging are described.