We present SKIRT, an efficient 3D Monte Carlo radiative transfer code, designed to treat continuum radiative transfer problems in dusty systems. In particular, we are using SKIRT to investigate the geometry, composition and formation of circumbinary dust disks around post-AGB stars. The main novelty in SKIRT Is the use of partly polychromatic photon packages, which allow a very efficient radiative transfer algorithm. We test the accuracy and the numerical requirements of the SKIRT code 13, comparing its results with recent ID and 2D benchmark calculations. We demonstrate that the common belief that Monte Carlo radiative transfer is slow, is not valid for state-of-the-art Monte Carlo codes where modern optimization techniques are included. Moreover, the very limited memory requirements of Monte Carlo radiative transfer make high-resolution three-dimensional radiative transfer simulations possible, while this might be more challenging for traditional. grid-based codes.