Cytogenetics analysis is at present the basic element of the diagnostic process of genetic disorders which are caused by chromosomal abnormalities. Since the chromosome banding technique has been introduced in the 1970s, it has been available as a diagnostic tool of a number of clinical syndromes. It enabled to prove the causal association between specific chromosomal abnormalities and clinical features observed in patients. However, since banding resolution is not always sufficient for the identification of chromosomal abnormalities, additional techniques for solving diagnostic dilemma of classical cytogenetics are needed. A new field of cytogenetics -- molecular cytogenetics, the product of a combination of cytogenetics and molecular biology, has increased the resolution and diagnostic utility of cytogenetic analysis. The basic method of molecular cytogenetics is fluorescence in situ hybridization (FISH). It enables a specific detection of unique sequences, chromosomal regions or entire chromosomes in metaphase, interphase cells or in tissue sections. In this article FISH technique and its modifications such as multicolor FISH (M-FISH, SKY, CCK), Primed In Situ Labelling (PRINS) and Comparative Genomic Hybridization (CGH) are presented. The range of applications and the use of these techniques for the identification of chromosomal abnormalities in relation to diagnostic possibilities of the classical methods of karyotyping is also discussed.