The results of investigations into in-plane photoconductivity in GaAs/Ga_0.7Al_0.3As multiple quantum wells are presented. The results indicate clearly that the experimental technique can be used as a reliable and sensitive tool to determine the purity of the material and the excitonic transitions that occur in the quantum well structures. It is shown that the photoconductivity is a strong function of the excitation intensity and the modulation frequency. The effects of DC background illumination and the application of longitudinal electric fields are also studied and the results are compared with a model based on hot-electron percolation in quantum well structures.