The book deals with a semiconductor nanostructure called symmetric rectangular double barrier. Complete and thorough analytical calculation of transmission coefficient of GaAs-AlGaAs symmetric rectangular double barrier (for E < V0) is presented. Values of energy for which resonant transmission peaks occur vary depending on height and width of each barrier and width of quantum well between the two barriers. We have numerically obtained the variations and presented them graphically as well as in the form of data tables. Some of the variations are surprising. The book is self-contained. It contains necessary background on Nanostructure Physics, Microelectronics and Quantum Mechanics to enable interested reader assimilate the book completely.