Stable telomere maintenance is essential for the indefinite cellular proliferation of germline and tumour cells. In most cases, telomere synthesis is performed by nucleoprotein enzyme complex of telomerase, that results in stabilisation of telomeres shortened to < or = 7 kb. Rarely, telomeres may be maintained via alternative (recombination-based) mechanism, which produces telomeres of heterogenous lengths (3-50 kb). Analysis of telomeres by in situ techniques, such as fluorescent in situ hybridisation (FISH) on metaphase spreads or on extended DNA fibres (fiber-FISH) and Primed in situ labelling (PRINS), enables to distinguish between these two mechanisms and to analyse individual telomeres in the given type of cells.