A description is given of the occurrence of fractional quantum Hall effects, and how these are influenced by the presence of disorder. It is shown that when electrons are photoexcited into a GaAs-GaAlAs heterojunction, the fractional state existing at a Landau level occupancy of 7/5 can dominate over that at 4/3. Activation energy measurements of the resistivity show how the energy gap of the fractional states is reduced by the presence of disorder. A new method of analysis of the resistivity minima is presented which is used to give information on the correlation length of the many body ground state.