The Y chromosome contains the largest nonrecombining block in the human genome. By virtue of its many polymorphisms, it is now the most informative haplotyping system, with applications in evolutionary studies, forensics, medical genetics, and genealogical reconstruction. However, the emergence of several unrelated and nonsystematic nomenclatures for Y-chromosomal binary haplogroups is an increasing source of confusion. To resolve this issue, 245 markers were genotyped in a globally representative set of samples, 74 of which were males from the Y Chromosome Consortium cell line repository. A single most parsimonious phylogeny was constructed for the 153 binary haplogroups observed. A simple set of rules was developed to unambiguously label the different clades nested within this tree. This hierarchical nomenclature system supersedes and unifies past nomenclatures and allows the inclusion of additional mutations and haplogroups yet to be discovered.[Supplementary Table 1, available as an online supplement at www.genome.org, lists all published markers included in this survey and primer information.]