Telomeres are the structures at the ends of linear chromosomes. In mammalian cells, they consist of hexanucleotide (TTAGGG) repeats, together with many associated proteins. In the absence of a compensatory mechanism, dividing cells undergo gradual telomere erosion until a critical degree of shortening results in chromosomal abnormalities and cell death or senescence. For T and B cells, the ability to undergo extensive cell division and clonal expansion is crucial for effective immune function. This article describes our current understanding of telomere-length regulation in lymphocytes and its implications for immune function.