Ab initio and semiempirical methods play a very important role in modern computational chemistry. The main advantage of semiempirical methods is a substantial reduction in the required computation time and, consequently, an increase in ability to execute calculations for large molecules. The disadvantages are the loss of the variational principle (one can obtain a total energy that is below the true total energy), the limited applicability (methods can only be applied to molecules containing elements that have been parameterized), and the danger of spurious results (especially for molecular systems that are different from those used for parameterizations). On the other hand, high-level ab initio techniques enable researchers to obtain very accurate results and act as a reference to lower-accuracy methods. These ab initio methods play an important role for calculations of small molecular systems and progress is being made to extend the area of application of such methods to bigger systems.