Exposure of hydrogenated amorphous silicon, a-Si:H, to light produces large-scale structural changes and increases the density of dangling Si bond defects acting as efficient carrier recombination centers. The latter is the well-studied Staebler-Wronski effect (SWE). All light-induced changes are metastable and disappear after annealing to approximately 200degreesC. This review focuses on one of the large-scale changes, namely that of the macroscopic density of the material. In all device quality materials, the initial stress is compressive with values typically in the range of 10(8)-10(9) Pa. Exposure to light produces additional compressive stress, which can exceed 2 x 10(7) Pa. The observed change of stress is due to a change of the volume of the unsupported material and not of its elastic modulus. The relative volume change, DeltaV/V, at 300 K becomes detectable at values in excess of about 10(-6) after only a few photons per Si atom have been absorbed. DeltaV/V saturates above 10(-3), under high-intensity light after an average of more than 10(6) photons per Si atom have been absorbed. DeltaV/V initially grows with t(0.50+/-0.04) under CW illumination producing carrier generation rate G in the range of 10(21) to a few 10(23) cm(-3) s(-1). The approach to saturation is well fitted by a stretched exponential function with stretch exponent close to 0.5. DeltaV/V is approximately proportional to G. The fastest and largest photo-expansion has been observed in the so-called "edge material" between the amorphous and microcrystalline state, produced by plasma enhanced CVD from increasingly diluted silane/hydrogen gas mixtures. The quantum efficiency of volume expansion has been observed to increase with the photon energy of the light in contrast to the SWE. No volume increase is observed in Ge rich a-Si1-xGex:H alloys and in hydrogenated microcrystalline material. Photo-expansion and the SWE show marked difference in spatial extend in the network, different evolution in time and different wavelength dependence. Hence, the two effects appear to be independent even though both involve hydrogen. (C) 2002 Elsevier Science B.V. All rights reserved.