Telomeres are specialized chromosomal end structures composed of repeat TTAGGG sequences in humans. They shorten with each cell division and thus serve as the "mitotic clock" of the cell. One of their main functions is the maintenance of chromosomal integrity and their excessive shortening is associated with DNA instability. Telomerase, a unique reverse transcriptase, is inactive in most somatic human cells and is up-regulated in most cancer cells. Recently, the biology of the telomere/telomerase system has attracted much attention because of its possible role in carcinogenesis and aging. In this article we review the biology of this system and its relevance to normal and malignant hematopoietic cells. The biological, diagnostic and prognostic value of telomere/telomerase biology is discussed, as well as its potential future applications in cancer therapeutics.