Sirtuins form a large homology family of enzymes found almost ubiquitously in living organisms and involved in numerous biological processes. The human genome encodes for seven paralog sirtuins, identified as SIRT1-7. In this review the major structural features of the sirtuin catalytic domain are illustrated and the relevant sources of biological information indicated. The multiple sequence alignment deduced from the optimal structural superposition of four human sirtuins having known three-dimensional structure, to which the amino acid sequences of the remaining three have been subsequently aligned, is also analyzed. The structure of the neighbor-joining tree deduced from the multiple sequence alignment, summarizing the evolutionary relationship among the member of the homology family is illustrated also in relation with the distinct catalytic activities detected in members this homology family.