After having analysed two types of unit operations (distillation and extraction) in a way that has not required a specific analysis of transport phenomena, this chapter begins a trilogy concerning such mechanisms, with reference to systems of interest for particular unit operations and chemical reactors. The first episode of the trilogy is dedicated to the transport of momentum, which is presented by focusing on the flow of a fluid inside a conduit (cylindrical, horizontal, with steady and one-dimensional flow). The concepts of laminar and turbulent motion are illustrated, also with the fundamental aid of the Reynolds number, and the general law of molecular transport is presented, which, applied to the momentum, becomes the Newton’s law on viscosity (for Newtonian fluids) and relates the viscosity to the shear stress and the radial gradient of axial velocity. For laminar motion, the radial field of both shear stress and velocity is obtained (this latter profile is then also described for turbulent motion). Finally, the definition of friction factor (and its relation to the Reynolds number) is used to show how, even in absence of information on the mean radial velocity, it is possible to argue the prevailing flow regime.