In pneumatic conveying, particles are generally in suspension in a turbulent gas stream. The question of drag on a single particle, and the effects of Reynolds number, particle shape and roughness, voidage, turbulence intensity and scale of turbulence, acceleration, etc. on drag are relevant to pneumatic conveying. These factors will be discussed in this chapter. Equations for calculating important properties such as drag coefficient, terminal velocity, minimum fluidization velocity, and the equation for flow through a packed bed are presented. The characteristics of a powder in terms of its fluidization behaviour are relevant to pneumatic conveying, and will also be discussed.