The book deals with an introductory idea of Leptonic and semileptonic decay of Heavy-Light mesons. It also highlight the application of a specific QCD potential model in this area of physics. Specifically, it also addresses the application of quantum mechanical perturbation method and explicitly the Dalagarno method of perturbation in this context. The necessity of modification of the model within the constraints of QCD and the model itself is explained and also improved accordingly. Out of the five chapters, the second and third chapters address the leptonic decay of B and D mesons whereas in the last two chapters, the study of semileptonic decay in terms of the universal form factor, Isgur-Wise function is well explained.