This presentation will review Paramagnetic Chemical Exchange Saturation Transfer (ParaCEST) MRI contrast agents. These agents should be thoroughly characterized with regard to their dependence on saturation time, saturation power, concentration, pH and temperature. Responsive ParaCEST agents can detect or measure enzyme activity, metabolites, metal ions, pH, redox state, temperature, and light. Some ParaCEST agents can also exhibit T2-Exchange relaxation. The intermediate exchange rate of a T2ex agent does not affect the T1 relaxivity of the agent. Therefore, the T2/T1 ratio of a T2ex agent can be employed to detect a biomarker.