Two-quasiparticle states in 166Er have been studied using the 167Er(d,t)166Er, 167Er(3He.α)166Er, 165Ho(3He,d)166Er, and 165Ho(α,t)166Er reactions. Beams of 15 MeV deuterons, 24 MeV 3He2+, and 27 MeV 4He2+ particles were produced by the McMaster University tandem Van de Graaff accelerator. The reaction products were analyzed with an Enge split-pole magnetic spectrograph and detected with photographic emulsions. Angular distributions were obtained for the (d,t) and (3He,d) reactions for levels up to ~2700 keV in excitation energy, although selected peaks were investigated at somewhat higher energies. The interpretation of the data was performed within the framework of the Unified model, incorporating pairing effects. In the neutron transfer study, two-quasiparticle states formed by removing a particle from the predominantly 7/2+ 167Er ground state were investigated, whereas the proton transfer study dealt with two-quasiparticle states formed by adding a particle to the 7/2− ground state of 165Ho. Several previous assignments have been supported and many new ones are proposed. Several other assignments proposed in an earlier proton transfer study have been found to be incorrect. Earlier observations that some levels were populated in both the neutron transfer and proton transfer reactions have been confirmed and extended. In addition, the observation in the (d,t) reaction of several states populated by l = 0 neutron transfers has been interpreted in terms of a complex mixing scheme involving the 7/2+ ± 1/2+ configurations.