Properties of quantum-well(QW) structures have proved useful in providing enhanced device characteristics and even in generating new device applications. This book presents a theory needed to understand the properties of QW structures, as well as the rules employed in the design of photodetectors based on QWs. A theoretical model is presented evaluating gain from the QW structure based on IV-VI semiconductor materials. The experimental and theoretical results concerning coupled QW structures are discussed. Semimagnetic quantum wells attract great attention due to potential device application in the field of spintronics. A study of three groups of semimagnetic structures is presented. Magnetoexcitons dispersion in coupled quantum wells are discussed using the Bethe-Salpeter formalism. Also presented are: high-resolution electron energy loss spectroscopy measurements on surface plasmon dispersion in systems exhibiting quantum well states; a review of different ways to achieve a spin splitting of two-dimensional electron and hole sub-bands; a review of two approaches to the quantisation of the particle motion in an infinite square; a study of the different disordered electron systems with various disorder strengths; and a study of quantum states of Double Quantum Wells and hydrogen-like molecules. An optical gain using the new material system InGaAsN for the operation of QW based semiconductor lasers is evaluated as well.