  We discuss the motion of colloidal particles relative to a two component
fluid consisting of solvent and solute. Particle motion can result from (i) net
body forces on the particle due to external fields such as gravity; (ii) slip
velocities on the particle surface due to surface dissipative phenomena. The
perturbations of the hydrodynamic flow field exhibits characteristic
differences in cases (i) and (ii) which reflect different patterns of momentum
flux corresponding to the existence of net forces, force dipoles or force
quadrupoles. In the absence of external fields, gradients of concentration or
pressure do not generate net forces on a colloidal particle. Such gradients can
nevertheless induce relative motion between particle and fluid. We present a
generic description of surface dissipative phenomena based on the linear
response of surface fluxes driven by conjugate surface forces. In this
framework we discuss different transport scenarios including self-propulsion
via surface slip that is induced by active processes on the particle surface.
We clarify the nature of force balances in such situations.
