In this chapter the ideas underlying the Boltzmann description of a fluid system are described. The classical Boltzmann equation is derived and the macroscopic quantities of mass, velocity and energy are defined in terms of the distribution function which describes the fluid. It is shown that the Boltzmann description of the fluid satisfies the fluid conservation equations. The form of the collision function is reviewed for a rare fluid, in which only binary collisions are considered, and for a simplified collision operator. An outline of the derivation of the Navier-Stokes equation and a discussion of the equilibrium distribution are given for the binary collision model.