Since the description of efficient electroluminescence from aluminium tris(hydroxyquinoline) in the mid 1980s, interest in new complexes and polymers with luminescence properties as emitting layers in light-emitting diodes has steadily increased. Recently, Ln(III) ion complexes have gained in importance for this type of application. Here we review some of the seminal work in the area, along with new developments in the field. The photophysical characterization of complexes of lanthanide ions both in solution and in the solid state allows the determination of which of the complexes might be successfully utilized as emitting layers in light-emitting diodes for display applications. However, the architecture of the device is also of major importance, to allow for good charge transport and recombination and thus obtain pure colors and high emission quantum efficiency.