The book contains thorough and complete analytical calculation leading to two transcendental equations satisfied by transverse wavevector dependent confined energy levels of the semiconductor nanostructure called isolated Quantum Well (QW). The book also contains thorough numerical calculation to bring out effects of the non-zero component of wavevector of electron parallel to interfaces of QW on parametric dependence of the confined energy levels. Complete explanations are presented of the results of numerical investigations using a transverse wavevector dependent effective potential. The book also deals with Physics of another semiconductor nanostructure called single rectangular tunnel barrier. Using a previously derived analytical expression, we have done novel numerical investigation of transverse wavevector dependence of transmission coefficient of single rectangular tunnel barrier and we have provided quantitatively exact explanation of the dependence. The book is self-contained; it contains necessary background on Quantum Mechanics, Microelectronics and Nanostructure Physics to enable readers assimilate the book completely.