The equations of motion used for linear acoustics are based on a number of assumptions that do not apply to turbulent flows. This chapter will discuss the classic theory of sound generation by turbulence, which is the basis for our understanding of the whole subject. Lighthill's acoustic analogy will be described and an inhomogeneous wave equation will be derived that identifies the sources of sound in an arbitrary unsteady flow. The solution to this wave equation in the presence of stationary surfaces will be given based on Curle's theorem and Green's functions. The concept of a tailored Green's function will also be introduced.