Relativistic bipolar outflows are common in the Universe when matter accretes onto compact objects, such as black holes. Their electromagnetic emission is extremely bright and spans a wide range of frequencies. I will begin by introducing accretion-powered astrophysical outflows and their importance as probes of some of the most extreme environments in the Universe. I will then discuss the physical mechanisms responsible for the acceleration of the jets to relativistic speed and for the release of radiation. Numerical modeling of jets is difficult due to their large dynamic range and extreme anisotropies caused by relativistic motions. Monte Carlo techniques are the weapon of choice to address such challenges. I will show the state of the art techniques and results and discuss future prospects for better understanding these extreme phenomena.