Abstract: This paper presents weighted importance sampling techniques for Monte Carlo form factor computation and for stochastic Jacobi radiosity system solution. Weighted importance sampling is a generalisation of importance sampling. The basic idea is to compute a-posteriori a correction factor to the importance sampling estimates, based on sample weights accumulated during sampling. With proper weights, the correction factor will compensate for statistical fluctuations and lead to a lower mean square error. Although weighted importance sampling is a simple extension to importance sampling, our experiments indicate that it can lead to a substantial reduction of the error at a very low additional computation and storage cost.