Interactions between electrons are likely to play an important role in certain transport properties of disordered semiconductors, particularly where these properties depend on electrons in localized states. A working classification of these interactions into intra-site interactions, inter-site interactions and polarization, is made. The first class is believed to affect primarily the transport properties o carriers around the mobility gap, by introducing two-electron wave functions into this region. The second class can introduce an activation energy into the d.c. hopping conductivity at very low temperatures, affect the thermo-electric power, and increase noticeably the a.c. hopping conductivity above a certain frequency. The third class may reduce the activation energy of the d.c. conductivity; a reduction to zero is theoretically possible. Instances where some of the above effects may have been observed are cited.