Over the past several years, the development of the science of chaos has led to new insights and understanding of nonlinear dynamics. Wave propagation can also become chaotic in both deterministic and stochastic (random media) environments. This problem has been treated by both ray and wave theory as well as through numerical simulations. In a ray description, chaotic propagation is characterized by exponential divergence of nearby rays with propagation distance (quantified by Lyapunov exponents). In a wave description, propagation is characterized by exponential divergence of narrow wave beams with range. In both cases, this divergence results in the tangling of the wave fields in phase space. In wave propagation through random media. the spreading due to chaotic wave propagation competes with spreading due to diffraction and scattering by small-scale irregularities, leading to different parameter regimes where these different spreading mechanisms dominate. Environments that produce strong multiple scattering by irregularities with a large length scale are most likely to cause chaotic wave propagation. Finally, implications and applications of chaotic wave theory along with remaining questions and issues will be discussed.