A hypothesis for ageing is proposed based on the properties of transposons. During the process of transposition, one copy of the DNA sequence generally remains at the same position while the other copy moves to another location in the genome. In this manner the DNA sequence of the transposon is effectively duplicated. With time the number of transposons increases exponentially and since in their new location they can inactivate an essential gene, they will eventually kill a cell line or organism. Thus transposons could be a cause of ageing. This hypothesis is attractive because it can explain many of the properties of senescent cells. Other processes capable of DNA self-duplication (e.g., reverse transcription) could also contribute to the increase in transposable DNA sequences.