This chapter begins with the principles determining a star’s structure: hydrostatic and thermal balance, and energy generation and transport. These imply that some stars have stably stratified cores and convective envelopes, whereas other stars have convective cores and stably stratified envelopes. The convection in stars is predominantly low Mach number, but the density at the top of a convection zone can be orders of magnitude smaller than the density at the bottom. We derive the anelastic equations which can model efficient, low Mach number convection. The properties of stars can be inferred by studying the waves at their surface. Here we describe sound and internal gravity waves, both of which have been observed in the Sun or other stars. The second half of this chapter discusses two phenomena at the interface between the convective and stably stratified layers of stars. First we consider convective overshoot, the convective motions which can extend into an adjacent stably stratified fluid. This can lead to substantial mixing in the stably stratified part of stars. Then, we discuss internal gravity wave generation by convection, which can lead to wave-induced energy or momentum transport. These illustrate some important fluid dynamical problems in stellar astrophysics.