This is an introduction to the branch of fluid mechanics concerned with the production of sound by hydrodynamic flows. It is designed for a one semester introductory course at the advanced undergraduate or graduate level. Great care is taken to explain underlying fluid mechanical and acoustic concepts, and to describe fully the steps in a complicated derivation. The discussion deals specifically with low Mach number flows, which enables the sound produced by `vortex-surface' interactions to be analyzed using the `compact Green's function'. This provides a routine procedure for estimating the sound, and an easy identification of those parts of a structure that are likely to be important sources of sound.