For many years researchers have understood the importance of the extracellular pH in solid tumors in relation to cancer morbidity and mortality. However current diagnostic imaging techniques do not allow for the non-invasive determination of pH in vivo. Recent research in the use of pH-responsive organic polymers for the preparation of imaging agents capable of imaging pH in vivo has demonstrated the tremendous potential of these materials in overcoming many of the problems associated with low molecular weight pH-responsive imaging agents. This review will highlight these recent developments with a focus on the use of pH-responsive polymers in the development of imaging agents for both fluorescent imaging and magnetic resonance spectroscopy and imaging. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1062-1067