Under the influence of different external stimuli condensed matter reveals its magnificent properties. The electric field, the temperature, the concentration gradients and the light are the basic "forces" responsible for processes such as the electrical, the thermal, the diffusion transport or optical phenomena. The action of the magnetic field brings about the galvanomagnetic or the thermomagnetic effects. New alloy semiconductors and the development of artificial semiconductor heterostructures led to the confinement of carriers in two, one or zero dimensions, opening a new window in condensed matter research. The application of a perpendicular magnetic field upon two-dimensional carriers, led to the discovering of astonishing phenomena, namely, the integer or the fractional quantum Hall effects and inspired radical theoretical interpretations. The reduced symmetry of low dimensional structures enhances decisively the role of the magnetic field orientation, bringing to light novel and unexpected phenomena. In the present book the effect of the application of an in-plane magnetic field upon low dimensional carriers, giving rise to impressive novel phenomena, is presented and discussed. Specifically, whenever a quantum well is subjected to an in-plane or tilted magnetic field, the elegant concept of Landau levels must be modified, because the carriers move under the competing influence of the Lorentz force and the force due to the quantum well confining potential. Under these conditions, the equal-energy surfaces or equivalently, the density of states (DOS), are qualitatively and quantitatively modified. The DOS diverges significantly from the ideal step-like two-dimensional carrier form. The book discusses various physical properties which are affected by the DOS modification.