Metal ions play numerous crucial roles in biology, and there is great interest in obtaining an accurate measurement of the intracellular concentrations of both tightly bound and exchangeable metal ions. Measuring the concentration of readily exchangeable transition metal ions in a cell has been particularly difficult because of the extremely small concentrations involved, interference from other metal ions and biomolecules, and the challenge of introducing probes into the cell with minimal perturbations. Recent work has made quantification of the intracellular exchangeable zinc pool possible for the first time using a cell-permeable, ratiometric, fluorescence resonance energy transfer based zinc biosensor.