PORTA is a radiative transfer code for the simulation of the spectral line intensity and polarization caused by scattering processes and the Hanle and Zeeman effects in 3D models of stellar atmospheres. The numerical method of solution is based on Jacobi’s iterative method and on a short-characteristics formal solver of the Stokes-vector transfer equation which uses monotonic Bézier interpolation. A crucial feature of PORTA is its parallelization strategy, which allows to speed up the numerical solution of complicated 3D problems by several orders of magnitude with respect to sequential radiative transfer approaches, given its excellent linear scaling with the number of available processors. PORTA can also be conveniently applied to solve the 3D radiative transfer problem of the intensity and polarization of the Sun’s continuous radiation.