Telomeric DNA generally comprises a repetitive, noncoding sequence which terminates in a single-stranded extension, the 3' overhang. Although these species-specific repeats show a large diversity in sequence and length, they comprise some specific characteristic conserved features. Therefore, analyses of the telomeric repeats of various organisms have delivered important insights concerning the origin and evolution of telomere structures. The canonical repeats are synthesized onto the 3' overhangs by the specialized telomerase enzyme. Although the general feature of the telomerase is to produce homogeneous repeats, variant repeats and irregular sequences are found in many species. In this chapter some aspects of the conserving and diverging evolutionary forces on the telomeric sequences are discussed. Proteins specifically binding to the telomeric sequences are of importance for nucleating the cap structure and they regulate telomere homeostasis and telomere end protection. Since a conserved binding specificity has been shown in several homologues of both single-stranded and double-stranded telomere-binding proteins, their binding may be considered a major selective force operating on telomeric DNA sequences.