This tutorial review highlights progress in the development of responsive magnetic resonance imaging (MRI) contrast agents for detecting and sensing biologically relevant metal ions. Molecular imaging with bioactivatable MRI indicators offers a potentially powerful methodology for studying the physiology and pathology of metals by capturing dynamic three-dimensional images of living systems for research and clinical applications. This emerging area at the interface of inorganic chemistry and the life sciences offers a broad palette of opportunities for researchers with interests ranging from coordination chemistry and spectroscopy to supramolecular chemistry and molecular recognition to metals in biology and medicine.