Telomeres, the extreme ends of the chromosomes play a key role in the process of cellular aging. Due to the 'end-replication-problem', successive shortening of the telomeres with each cell division results in a mitotic clock and it was shown in vitro that this clock limits the replicative capacity of cell proliferation. Telomerase counteracts telomere erosion and provides some somatic cells an unlimited proliferative potential in vitro. The present views of telomeres and telomerase functions in cellular aging in vitro are presented. Possibilities and limitations in the evaluation of the in vivo impact of telomere erosion on human aging, aging syndromes and age related diseases are reviewed. Unresolved questions, future experimental approaches and emerging therapeutic applications are discussed.