The present state of a-Si:H solar cells and studies on the light-induced effects in a-Si:H films and a-Si:H solar cells are presented. The conversion efficiencies attained are 11.5%, 10.1% and 7.5% (8.5% in active area) for 1.0cm2 singlejunction, multijunction cells and 1200cm2 submodule, respectively. The density of light-induced defects, ANs, changes with deposition temperature and annealing temperature. This suggests a strong correlation between the ANs and the network structure in a-Si:H films. Field testing results indicate that the degradation of a-Si:H modules can be suppressed within 10% of the original efficiency after 2 years operation. It is also indicated that the multijunction cells are more stable to light exposure than single p-i-n cells.