We have studied Physics of a Nanostructure called Quantum Well (QW). We have numerically calculated confined energy levels of electrons, light holes and heavy holes in isolated GaAs-AlGaAs QWs of various widths and depths. We have presented both graphically as well as in the form of data tables variation of the energy levels as width and depth of QW are varied. We find that the values and variations of energy are exactly the same as the values and variations of energy of resonant transmission peaks of symmetric rectangular double barrier. Surprisingly, this is exact numerical agreement. The agreement remains exact even if we take effects of effective mass inequality into account. The book is self-contained; it contains necessary Nanostructure Physics, Microelectronics, Quantum Mechanics, as well as results for symmetric rectangular double barrier for ready reference. Interested reader will be able to assimilate the book completely.