AbstractLanthanide complexes have attracted much attention in the biomedical field, and today various imaging applications make use of their versatile magnetic and luminescence properties. In this minireview, we give insight into the mechanistic aspects that allow modulation of the relaxation or chemical exchange saturation transfer (CEST) features, and thus the magnetic resonance imaging (MRI) efficiency of paramagnetic lanthanide chelates in order to create agents that are capable of providing an MRI response as a function of a specific biomarker. We focus on the detection of neurotransmitters, enzymatic activities, and amyloid peptides. We also describe two selected theranostic strategies: 1) a novel approach directed at monitoring drug release from liponanoparticles and 2) molecular or nanoparticle probes for the MRI visualization of photosensitizer delivery in photodynamic therapy.