Alu insertional elements, the most abundant class of SINEs in humans are dimeric sequences approximately 300 bp in length derived from the 7SL RNA gene. These sequences contain a bipartite RNA pol III promoter, a central poly A tract, a 3' poly A tail, numerous CpG islands and are bracketed by short direct repeats. An estimated 500,000 to 1 x 10(6) units are dispersed throughout the human haploid genome primarily in AT rich neighborhoods located within larger GC dense chromosomal regions via a mechanism known as retroposition. Retroposition activity of Alu elements is determined by both internal and flanking regulatory elements as well as distant genes affecting transcription or transcript stability. Alu elements impact the organization and expression of the human genome at many levels including the processes of recombination, transcription and translation. Twelve subfamilies of Alu are defined by distinct patterns of diagnostic base substitutions. Subfamilies may be classified as young, intermediate or old reflecting the time since the start of retroposition by their members. Some insertions of the youngest subfamilies are not yet fixed in the human species and represent polymorphic loci. Alus are excellent molecular markers for a variety of reasons. They aid in tracing the complex pattern of duplication and rearrangements that occurred during the evolution of primate genome. Unlike other mutations, Alu sequences are rarely lost completely once retroposed, have a defined ancestral state and are free from homoplasy since independent and identical insertions are highly unlikely. Because of these characteristics, Alus are literally molecular fossils. Polymorphic Alu loci are especially useful in studies of human genetic diversity and in pedigree and forensic analysis.