Magnetic Resonance Imaging is a diagnostic method used to image organs and soft tissue in the body. The instrument relies on the use of magnets and radiofrequency radiation to examine the proton nuclei of water within tissue and organs. Through use of contrast agents, the images acquired can have improved detail which is critical for diagnoses of tumors, inflammation, and other abnormalities. These agents utilize paramagnetic metals to induce magnetic effects on the protons, enabling a change in proton relaxivity. Early contrast agent development involved use of gadolinium (III) transition metal complexes. Gadolinium-based contrast agents work well to improve contrast but are retained in the body indefinitely. The long-term use of gadolinium is not fully understood. This prompted developments of a new generation of contrast agents. These agents use endogenous, first-row transition metals such as Mn(II) and Fe(III). The contrast agents highlighted in this seminar exhibit unique strategies for contrast imaging such as site-specificity, MRI response modulation based on pH- and concentration- effects on paramagnetism, and redox-active MRI signaling in response to interactions with reactive oxygen species.