Mixing in powders generally results from relative motion of groups of particles – convective mixing – or of individuals – diffusive mixing. Segregation or demixing occurs when the motion of individual particles is biased according to their particular characteristics – size, shape, composition etc. In the absence of such bias, individual motion invariably leads to homogenization of the mixture. Relationships between mixing/segregation processes and the external and interparticle forces responsible for causing or opposing relative motion are reviewed. Specific examples of mixing and segregation in flow over surfaces, in rotating cylinders and other applications are described.