A number of biological roles have been proposed for quadruplex, also referred to as G4 or tetraplex, DNA. The presence of quadruplex DNA may lead to errors in some biological processes and be required in others. Proteins that interact with quadruplex DNA have been identified including those that cause Bloom's and Werner's syndromes. There are small molecules that specifically bind to quadruplex DNA, inhibit telomerase, and are cytotoxic towards tumor cells indicating a role for quadruplex DNA in telomere function. It is now possible to make testable proposals for the possible biological implications of quadruplex DNA in replication, transcription, and recombination as well as possible routes to therapeutic intervention.