This chapter introduces the so-called shallow-water approximation, which allows for a simplified treatment of motions with horizontal spatial scales much larger than the vertical one. After dealing with a single layer of homogeneous fluid, which limits the dynamics to the barotropic case, we move on to a multilayer fluid, capable of describing baroclinic motions. In this context, the discussion of a simulation of a significant oceanographic phenomenon allows the introduction of the concepts of intrinsic oceanic variability and chaotic dynamical system.