MR imaging and MR spectroscopy are fundamental techniques for molecular and cellular imaging. The implementation of molecular imaging techniques for MR imaging has been facilitated by the development of targeted contrast agents for detection of gene expression, surface receptors, or enzyme activity. This article reviews recent advances in targeting strategies for MR molecular imaging focusing on the two main classes of MR contrast agents, paramagnetic gadolinium chelates and superparamagnetic iron oxide compounds, including methods for improving MR contrast through modulation of relaxivity in response to specific biologic interactions.