We have demonstrated Physics for which we need to modify boundary condition to be used in matching solutions of Schroedinger equation at finite potential discontinuities i.e. at interfaces between barrier and quantum well materials, because of difference of effective mass of electron in their conduction bands. Using the modified boundary condition, we have presented thorough and complete analytical calculation of transmission coefficient of nanostructures called single rectangular tunnel barrier and symmetric rectangular double barrier. We have presented numerical investigations for the two nanostructures. We find that parametric dependences of energy of resonant transmission peaks of the double barrier do not change qualitatively because of taking effective mass inequality into account; but we find changes in energy and in full width at half maxima (FWHM) of the resonant peaks. The book is self contained; it contains necessary Nanostructure Physics, Microelectronics and Quantum Mechanics to enable interested reader assimilate the book completely.