The chapter discusses the refraction of acoustic-gravity waves in the solar atmosphere, where sound speed changes little with height, leading to minimal bending of phase and group velocities. In contrast, gravity waves show strong refraction due to changes in the frequency $\omega_{\mathrm{BV}}$, with phase velocity bending away from the vertical and group velocity bending towards it. As the frequency decreases, both $u_{\mathrm{g}}$ and $w_{\mathrm{g}}$ approach zero. The section on shock waves explains that small-amplitude disturbances propagate adiabatically and are slowly damped. As amplitude increases, nonlinear effects become significant, leading to shock formation. Shocks move supersonically through the fluid, outrunning preshock disturbances and persisting until damped by dissipative mechanisms. The material behind a shock is hotter, denser, and has higher pressure and entropy than the material in front. Strong shocks, like those in supernova explosions, blow away the outer envelope of a star. The development of shocks is further explored, showing that in a one-dimensional, adiabatic flow, the continuity and momentum equations can be solved to describe the propagation of a pulse. The solution reveals that the wave front steepens into a shock, with all variables changing abruptly across a thin layer. The shock thickness is determined by the Knudsen number, which is very small in stellar atmospheres. For steady shocks, the conservation laws and jump relations are derived, showing that the upstream and downstream properties of the shock are related by specific equations. The entropy jump across a shock is always positive, indicating irreversible dissipation of energy. Rarefaction discontinuities are mechanically unstable and disintegrate immediately, while compression shocks are stable and can propagate as sharp discontinuities. Relativistic shock waves are also discussed, with jump conditions derived in a relativistic frame. The structure of shock fronts is analyzed, considering both viscous and conducting fluids. Viscous shocks have a monotonic variation in velocity, density, pressure, and temperature, while conducting shocks can be isothermal for strong shocks.The chapter discusses the refraction of acoustic-gravity waves in the solar atmosphere, where sound speed changes little with height, leading to minimal bending of phase and group velocities. In contrast, gravity waves show strong refraction due to changes in the frequency $\omega_{\mathrm{BV}}$, with phase velocity bending away from the vertical and group velocity bending towards it. As the frequency decreases, both $u_{\mathrm{g}}$ and $w_{\mathrm{g}}$ approach zero. The section on shock waves explains that small-amplitude disturbances propagate adiabatically and are slowly damped. As amplitude increases, nonlinear effects become significant, leading to shock formation. Shocks move supersonically through the fluid, outrunning preshock disturbances and persisting until damped by dissipative mechanisms. The material behind a shock is hotter, denser, and has higher pressure and entropy than the material in front. Strong shocks, like those in supernova explosions, blow away the outer envelope of a star. The development of shocks is further explored, showing that in a one-dimensional, adiabatic flow, the continuity and momentum equations can be solved to describe the propagation of a pulse. The solution reveals that the wave front steepens into a shock, with all variables changing abruptly across a thin layer. The shock thickness is determined by the Knudsen number, which is very small in stellar atmospheres. For steady shocks, the conservation laws and jump relations are derived, showing that the upstream and downstream properties of the shock are related by specific equations. The entropy jump across a shock is always positive, indicating irreversible dissipation of energy. Rarefaction discontinuities are mechanically unstable and disintegrate immediately, while compression shocks are stable and can propagate as sharp discontinuities. Relativistic shock waves are also discussed, with jump conditions derived in a relativistic frame. The structure of shock fronts is analyzed, considering both viscous and conducting fluids. Viscous shocks have a monotonic variation in velocity, density, pressure, and temperature, while conducting shocks can be isothermal for strong shocks.