Efforts to expand the genetic alphabet are predicated upon a stable and replicable third base pair. Recent progress has resulted in the development of several candidates that are both stable in duplex DNA and replicated by DNA polymerases with various degrees of efficiency and fidelity. The candidate base pairs draw upon unnatural hydrogen-bonding topologies as well as upon shape complementarity and hydrophobic forces. This review provides a critical comparison of the third base pair candidates and discusses the further work required to expand the genetic alphabet.