Semiclassical approach to states near potential barrier top

16 pages, 10 eps figures, Revtex, submitted to Phys. Rev. AWithin the framework of the instanton approach we present analytical results for the following model problems: (i) particle penetration through a parabolic potential barrier, where the instanton solution practically coincides with the exact (quantum) one; (ii) descriptions of highly excited states in two types of anharmonic potentials: double-well $X^4$, and decay $X^3$. For the former case the instanton method reproduces accurately not only single well and double-well quantization but as well a crossover region (in the contrast with the standard WKB approach which fails to describe the crossover behavior), and for the latter case the instanton method allows to study resonance broadening and collapse phenomena. We investigate also resonance tunneling, playing a relevant role in many semiconducting devices. We show that in a broad region of energies the instanton approach gives exact (quantum) results. Applications of the method and of the results may concern the various systems in physics, chemistry and biology exhibiting double level behavior and resonance tunneling