Number of particles in an organ or region is a valuable data as well as the volume. Many techniques have been developed for the estimation of total number or numerical density of cells in tissues and organs. The quality and reliability of each method has constantly been increased compared to its predecessors. Numerical density is one of the parameters that are used to assess the association between a structure and its function. It should be kept in mind that reliability of biological comments derived from the data is dependent on the method that is used for quantitative estimation of parameters. As stated previously, one of the most important morphometric parameters in biological studies is the 'number'. Since the 'number' has no-dimensions (i.e. is not bounded by any dimensional property) it could not be directly estimated using a 2D section plane. A good alternative is to use a "volume probe" generated by two consecutive sections, which is called the disector. The disector counting method that was developed by Steno in 1984 is the most efficient and unbiased solution for particle counting. In this review, we will summarize the basic principles of the disector approach and give some examples focused especially on some basic topics in neuroscience research.