To serve well as luminescent labels in biological applications, complexes of the lanthanide ions need to fulfil many criteria, including the requirement of high quantum yields, stability towards dissociation in biological media and preferably, excitation at wavelengths over 350 nm. Herein, a number of aspects regarding the sensitization of europium(III) and terbium(III) ions are discussed with respect to the design and development of novel complexes. The effect of the addition of anions such as fluoride and phosphate to the Eu(III) and Tb(III) complexes of a known tris(bipyridyl) cryptate, are studied with a view to gaining a better understanding of non-radiative deactivation pathways of the lanthanide excited state. Enhancements of Eu3+ emission within the cryptate are noted whilst with the Tb3+ analogue, emission is significantly reduced upon displacement of OH by these ions. A new ligand based on a polyaminocarboxylate structure with a single bipyridyl chromophore is then reported, which displays nine-coordination around the enclosed lanthanide ion. The luminescence properties of the europium complex are very good but the terbium complex shows more complex behaviour, suggestive of a back energy transfer mechanism from the terbium excited state to the ligand triplet level. The development of ligands with mixed N,O pyrido-phenol chromophoric groups, which are expected to be better sensitizers of terbium(III) luminescence, is also outlined. Finally, in the quest to sensitize europium using longer wavelengths, acridone is incorporated into an azamacrocyclic structure and luminescence is observed from the complexed Eu3+. The quantum yield of luminescence is found to increase with decreasing distance between the sensitizer and the ion, and in the presence of coordinating donor atoms.