▪ Abstract Using luminescent lanthanides, instead of conventional fluorophores, as donor molecules in resonance energy transfer measurements offers many technical advantages and opens up a wide range of new applications. Advantages include farther measurable distances (100 Å) with greater accuracy, insensitivity to incomplete labeling, and the ability to use generic relatively large labels, when necessary. Applications highlighted include the study of ion channels in living cells, protein-protein interaction in cells, DNA-protein complexes, and high-throughput screening assays to measure peptide dimerization associated with DNA transcription factors and ligand-receptor interactions.