Magnetic resonance imaging (MRI) is a popular imaging technique in medical diagnostics. With the development of contrast agents, interest in its applications has grown tremendously. Significant effort has been made in order to identify the most important parameters that enhance the relaxation efficiency of MRI probes. Taking into account the requirements for an optimal magnetic performance, different contrast agents have been synthesized and studied. Moreover, novel bimodal probes have been developed in order to exploit the high sensitivity and resolution of optical microscopy with the ability of MRI to image opaque samples. Employing this strategy enables the simultaneous visualization of the same biological structures at different resolutions and depths. Throughout this review, different approaches used to improve relaxivity, especially by increasing the molecular volume and hence the rotational tumbling time of the agent, are highlighted. Several ways to obtain bimodal contrast agents are discussed in detail. Finally, lanthanide complexes incorporating an aromatic unit permitting efficient sensitization of lanthanide luminescence in combination with the relaxometric properties of gadolinium analogues are listed.