This chapter summarizes the magnetic resonance studies of chemical exchange and describe the basic mechanism for chemical exchange image contrast using the Bloch equations to relate the contrast to the concentration of the agent and its exchange and a relaxation properties. The imaging of cells with magnetic resonance imaging requires some magnetic labeling strategy that allows the discrimination between labeled cells and their surroundings. The chemical exchange saturation transfer contrast mechanism is a very promising new tool for studying biological phenomena. Endogenous small metabolites and a larger biological macromolecules can be highlighted using the saturation transfer imaging sequence, and two new methods have been developed to measure their exchange rates. The chapter describes some of the unique features of these magnetic resonance imaging agents, which include the ability to label cells with different frequencies to a distinguish between these cells.