Telomeres are structurally and functionally complex. They consist of an array of simple DNA repeats at the extreme end of the chromosome with a more complex array of repeats adjacent to it. A large number of proteins have been identified that bind to the telomeric DNA repeats or to the protein complexes that are built at the chromosome end. Telomeres tend to form associations with each other. These associations have been implicated in the formation of nuclear domains that may be important for transcriptional regulation, for sister chromatid pairing at mitosis, and for homologous meiotic synapsis. Telomeric chromosome ends do not cause delays in cell cycle progression, nor are they subject to DNA repair as are broken chromosome ends. Telomeres also provide a separate mechanism for adding additional copies of the telomeric DNA to chromosome ends. This is needed to counterbalance the loss of DNA sequences from chromosome ends due to incomplete DNA replication. The components that participate in the latter mechanism and this process have been characterized in detail; the other functions of telomeres are less well understood but are the subjects of active investigation.