Lanthanide complexes are more and more used in biomedical imaging as contrast agents (CA). The development of these paramagnetic complexes as CA for medical magnetic resonance imaging (MRI) and luminescent probes for optical imaging is very complementary. Gd complexes are well known as CA for MRI and Eu/Tb complexes are often used in microscopy or fluorescence imaging. Each imaging technique has its limitation: low sensitivity but high spatial resolution for MRI and limited penetration but high sensitivity for optical imaging. A bimodal agent can be used for these two methods and give more informations, they can be visualized simultaneously by light and MR imaging. Such compounds are based on the coordination chemistry of the lanthanide ions with an organic ligand to form a stable complex and on the properties of the lanthanide ions. Gd complexes with a chromophore allows also the luminescent detection. This review describes the properties of the lanthanide ions and of their complexes and gives some typical applications of the complexes. The luminescence properties show high quantum yield and long luminescence lifetimes. The relaxometric data of the Gd complexes are comparable or higher than commercial and clinically Gd-DTPA derivatives.