The freely accessible YHRD (Y Chromosome Haplotype Reference Database, www.yhrd.org) is designed to store Y chromosome haplotypes from global populations and had replaced three earlier database versions collecting European, Asian and US American Y chromosomes separately. The focus is to disseminate haplotype frequency data to forensic analysts, researchers, and to everyone who is interested in historical and family genetics. YHRD considers reduction of the available number of polymorphisms on the Y chromosome to a uniform data string of 11 highly variable Y-STR loci as an efficient way to rapidly screen many world populations and to make their Y chromosome profiles comparable. Typing of the YHRD 11-locus core set is facilitated by commercial products, namely diagnostic PCR kits, and endorsed by scientific and forensic analyst's societies as ISFG or SWGDAM. YHRD is structured by the assignment of each submitted population sample to a set of populations sharing a common linguistic, demographic, genetic or geographic background (metapopulations). This principle facilitates the statistical evaluation of haplotype matches due to a significant enlargement of sample sizes. With the rapid growth of the database the definition of homogeneous metapopulations is now also feasible solely on the basis of the genetic data as exemplified for the whole dataset of YHRD, release 19 (August 2006). Large sample numbers within genetically defined metapopulations also allows the development of biostatistical methods to estimate the frequency of unobserved or rare haplotypes ("haplotype frequency surveying method"). Essential for the YHRD project is its collaborative character relying on the engagement of individual laboratories to make their data accessible via YHRD and to share the YHRD standards regarding data quality.