Many physiological processes are governed by small reactive species, anions and cations. As such, many pathologies are also marked by substantial deviations in the concentration and distribution of these species. Molecular probes have proven invaluable tools at deciphering the roles of biologically-relevant species, both in vitro and in vivo. Nonetheless, despite advances in molecular imaging, many small cations, anions and reactive species remain difficult to image selectively under conditions relevant to physiological processes. This difficulty is even more accrued in vivo, where the burgeoning field of responsive molecular imaging probes for MRI has yet to surmount its crucial problems of insufficient sensitivity and poor distribution. We have addressed the challenges of both in vitro and in vivo imaging by exploiting the unique coordination chemistry and physical properties of lanthanides. Luminescent lanthanide complexes such as EuIII and TbIII enable the detection of metals such as potassium and copper, and anions such as phosphate and ATP selectively.  A case study with copper will demonstrate how we translated the lessons learned from these luminescent probes to responsive MRI contrast agents for in vivo imaging of metal ions.