Manganese is a widespread element, essential for human health. However, over-exposure to manganese can be harmful, especially to the nervous system. Optical assays for Mn(II) are therefore of interest as environmental tools as well as for biological studies. We have developed a supramolecular displacement assay for Mn(II) involving a chelating dye, a colorless chelator, and a reporter ion. This assay reveals the presence of Mn(II) with a visible color change. As a result of a unique combination of properties, Mn(II) has been employed as a tool to study calcium channels and brain function. For studies of Mn(II) transport in tissues, fluorescent assays are of great utility. Previous fluorescent probes for Mn(II) have involved fluorophores that are quenched as a result of interaction between the fluorophore and unpaired electrons in the metal. A similar displacement assay involving two chelating fluorophores and a reporter ion provides an �on-fluorescence� signal. The method was shown to give ratiometric fluorescence in solution and in cells. Ongoing work is aimed at optimizing Mn(II) selectivity properties of the chelating fluorophores among other challenges.