This chapter presents an overview of the performance of current multilevel methods for computational thermochemistry and thermochemical kinetics. Multilevel methods extrapolate to the exact solution of the electronic Schödinger equations by using calculations carried out with two or more levels, where a level is a combination of a specific form for the many-electron wave function and a specific one-electron basis set. This chapter compares the performance for thermochemistry and thermochemical kinetics of several multilevel methods, including scaling-all-correlation (SAC), complete basis set (CBS) methods, multi-coefficient correlation methods (MCCM), and Gaussian-3 extended (G3X) methods. It also compares these methods to hybrid density functional theory, and additional calculations are presented to test the importance of diffuse basis functions on hydrogen. In order of decreasing cost, the G3SX(MP3), MCG3/3, MC-QCISD/3, CBS-4, mPW1PW91/MG3S, and SAC/3 methods are shown to provide especially good performance-to-cost tradeoffs.