Tetrathiafulvalene (TTF)-based ligands and lanthanide ions have been intensively used for their electronic conductivity and optical properties, respectively. Their combination leads to a new class of coordination compounds that are able to display single-molecule magnet (SMM) behavior. Magnetic bistability resulting of such behavior could find potential applications in high-density data storage and quantum computing. In this chapter, a library of TTF-based magnets containing lanthanide ions is presented. Among this series, the influence of the coordination sphere and intra- and intermolecular interactions such as exchange, dipolar, supramolecular, and hyperfine interactions is probed through molecular engineering, magnetic dilutions, and isotopic enrichment.