The discovery of DNAzymes that can catalyze a wide range of reactions in the presence of metal ions is important on both fundamental and practical levels; it advances our understanding of metal-nucleic acid interactions and allows for the design of highly sensitive and selective metal ion sensors. A crucial factor in this success is a technique known as in vitro selection, which can rapidly select metal-specific RNA-cleaving DNAzymes. In vitro selection is an iterative process where a DNA pool containing a random region is incubated with the target metal ion. Those DNA sequences that catalyze the preferred reaction (the "winners") are amplified and carried on to the next step, where the selection is carried out under more stringent conditions. In this way, the selection pool becomes enriched with DNAzymes that exhibit desirable activity and selectivity. The method described can be applied to isolate DNAzymes selective to many different types of metal ions or different oxidation states of the same metal ion.