Summary: The study of plasma under extreme conditions of density, such as those found in dense stellar objects or in laser-compression schemes associated with inertial confinement fusion, requires the inclusion of quantum-mechanical and even special-relativistic effects. In particular, the Fermi pressure of denegerate electrons plays a significant role in the dynamics of ultra-dense plasmas. In this thesis, a variety of models for nonlinear waves in low-temperature quantum plasmas are investigated by using a multifluid hydrodynamical formalism for plasmas composed of electrons, ions and, in two cases, positrons. Criteria for the existence and stability of nonlinear structures are found using the method of multiple scale perturbation and Sagdeev’s pseudopotential method.