In order for luminescent lanthanide complexes to be used in fluoroimmunoassays, a number of criteria must be adhered to, such as high quantum yield, high stability in aqueous media and insensitivity to the surrounding environment. In this thesis, a number of new ligand systems, designed with regard to achieving novel sensitising ligands of europium and terbium for use in cellular assays are discussed. A new ligand based upon a macrocyclic polyaminocarboxylate incorporating a single bipyridine chromophore and its complexes of europium and terbium are reported. Luminescence properties of both complexes are very good, with the terbium analogue displaying more complex behaviour, which suggest a back energy transfer mechanism from the emissive metal centre to the bipyridine triplet. The development of a different ligand incorporating the bipyridine chromophore as the binding groups based on a macrocyclic backbone are also reported. Simple intramolecular europium and terbium complexes of salicylaldehyde and ohydroxybenzophenone were prepared and their photophysical properties recorded. These chelates showed poor stability in solution which led to the investigation of azacrown appended version of these ligands with the aim to improve stability. Europium chelates of these sensitisers displayed poor emissive properties at room temperature postulated to be due to a ligand to metal charge transfer deactivation mechanism. Finally, with the aim of developing charge neutral, fat-soluble luminescent lanthanide complexes capable of cell loading, donors of mixed pyrido-phenol sensitisers were developed. The luminescent properties were found to be poor for the europium complex due to ligand to metal charge transfer. Terbium complexes of these ligands showed sensitivity towards oxygen and temperature, indicating the presence of back energy transfer from the metal centre to the aryl triplet. Complexes of 8- hydroxyquinoline with europium are also reported.