The photoconductive properties of undoped hydrogenated amorphous silicon have been extensively studied. Measurements of optical absorption, dark d.c. conductivity, steady state photoconductivity, step response transient rise photoconductivity and impulse response flash decay photoconductivity have been made. In addition, computer simulation has been used to give an insight into the physical processes involved in the photoconductivity experiments.Two materials were used in the study, to provide a comparison. All the above measurements, except the transient measurements were made on sputtered material prepared by the author. a-Si:H prepared elsewhere by the glow discharge decomposition of silane was measured by all the above experiments, except optical absorption.The results obtained from the flow discharge material were interpreted as due to recombination in distributed states, which are restricted in energy, extending from the dark Fermi level upwards to 0.6eV below Ec. The capture cross section of these states was of a value expected for the neutral dangling bond, so they have been denoted as Do states. States outside this energy were seen to be ineffective as recombination centres. The model thus has features intermediate between a simple 2-trap system and a distributed density of states. Chapter 2 presents a detailed analysis of steady state photoconductivity for the case of a single correlated defect level, and demonstrates its near equivalence to a simple 2-defect one electron system. This partly justifies the subsequent use of one electron states in computer modelling. Computer simulation successfully predicts the form of the experimental step response. The flash decay required the existence of an extra discrete state at 0.4eV, but gave a better quantitative fit.The sputtered material appeared to have the recombination in the steady state controlled by discrete states 0.6eV below Ec, but inconsistencies remain between the interpretations of different measurements on this material.