In magnetic resonance imaging (MRI), contrast between different tissues depends on a number of parameters including transversal and longitudinal relaxation times (T1, T2), susceptibility, proton density and flow. Despite this multiparameter dependence, the usefulness of contrast agents has been recognized since MRI was first introduced. Contrast agents in MRI are chemicals which by virtue of their unpaired electrons induce relaxation in the water molecules in their vicinity. Depending on their chemical structure, different contrast agents may alter the signal in specific regions of the image or in specific pathologies. Established and widely used contrast agents are small Gd chelates which, after injection into the bloodstream, rapidly extravasate into the extravascular space. More recently, a different class of contrast agents has been developed with the property of remaining in the vascular space (blood pool contrast agents). Blood pool contrast agents have been used to investigate vasculature in tumors (angiogenesis) and in the brain (cerebral blood volume and functional MRI). In the last few years, great attention has been devoted to the use of contrast agents for cellular and molecular imaging. It has been realized that cells, opportunely marked with contrast agents, can be visualized in vivo with MRI. The technique has been applied successfully to in vivo detection of stem cells after transplantation. Moreover, contrast agents have been applied to imaging phenomena at the molecular level in vivo, for instance to investigate gene expression in tissues. In this article we describe some innovative applications of contrast agents for MRI.