This book is devoted to asymptotic analysis of solutions of second order ordinary differential equations with a small parameter. The main emphasis is on various constructive schemes of obtaining asymptotic solutions, their advantages and drawbacks, and specific computations. The author gives a complete overview of the state of the theory and also concentrates on some lesser known aspects and problems, in particular the problems in which exponentially small terms should be taken into account or the analysis of equations with close transition points. Such applications as the derivation of the formulas for the quasiclassical quantizations, spectrum splitting in a symmetrical potential, etc., are considered.