Telomeres are structures of tandem TTAGGG repeats that are found at the ends of chromosomes and preserve genomic DNA by serving as a disposable buffer to protect DNA termini during chromosome replication. In this process, the telomere itself shortens with each cell division and can consequently be thought of as a cellular 'clock', reflecting the age of a cell and the time until senescence. Telomere shortening and changes in the levels of telomerase, the enzyme that maintains telomeres, occur in the context of certain somatic diseases and in response to selected physical stressors. Emerging evidence indicates that telomeres shorten with exposure to psychosocial stress (including early-life stress) and perhaps in association with some psychiatric disorders. These discoveries suggest that telomere shortening might be a useful biomarker for the overall stress response of an organism to various pathogenic conditions. In this regard, telomeres and their response to both somatic and psychiatric illness could serve as a unifying stress-response biomarker that crosses the brain/body distinction that is often made in medicine. Prospective studies will help to clarify whether this biomarker has broad utility in psychiatry and medicine for the evaluation of responses to psychosocial stressors. The possibility that telomere shortening can be slowed or reversed by psychiatric and psychosocial interventions could represent an opportunity for developing novel preventative and therapeutic approaches.