This special issue of Oceanography presents a survey of recent work on internal waves in the ocean. The undersea analogue to the surface waves we see breaking on beaches, internal waves play an important role in transferring heat, energy, and momentum in the ocean. When they break, the turbulence they produce is a vital aspect of the ocean's meridional overturning circulation. Numerical circulation models must parameterize internal waves and their breaking because computers will likely never be powerful enough to simultaneously resolve climate and internal wave scales. The demonstrated sensitivity of these models to the magnitude and distribution of internal wave-driven mixing is the primary motivation for the study of oceanic internal waves. Because internal waves can travel far from their source regions to where they break, progress requires understanding not only their generation but also their propagation through the eddying ocean and the processes that eventually lead to their breaking. Additionally, in certain regions such as near coasts and near strong generation regions, internal waves can develop into sharp fronts wherein the thermocline dramatically shoals hundreds of meters in only a few minutes. These "nonlinear" internal waves can have horizontal currents of several knots (1 knot is roughly 2 meters per second), and are strong enough to significantly affect surface navigation of vessels. Vertical current anomalies often reach one knot as well, posing issues for subsurface navigation and engineering structures associated with offshore energy development. Finally, the upwelling and turbulent mixing supported by internal waves can be vital for transporting nutrient-rich fluid into coastal ecosystems such as coral reefs. Below, we provide a very brief introduction to some of the central concepts discussed in the 14 articles that make up the special issue section, and then put each of these articles in context.