In this paper we discuss the performance of colloidal silica antireflection coatings which have been developed for use in high-power KrF and Nd:glass lasers. These coatings have reproducibly given transmissions of more than 99.8% per surface and have exhibited laser damage thresholds as high as 20 J/cm(2)for l-ns pulses at 1.06,microm and 8 J/cm(2)for 10-ns pulses at 0.25,microm. These damage thresholds are a factor of between 2 and 3 better than values obtained with evaporated oxide AR coatings. Novel multilayer coatings incorporating colloidalsilica as the low-index layer are also described. One such coating gave more than 50% reflectivity for a two-layer coating at normal incidence.