We present MONKI (Monte Carlo KNMI), an accurate and efficient Fortran radiative transfer code for simulating total and polarised sunlight in 3D planetary atmospheres. Its novelty is the derivation of Monte Carlo equations for the transfer of polarised light, fully accounting for polarisation-dependent scattering directions. This formulation gives reliable convergence even for optically thick, strongly polarising atmospheres, supporting observations of Earth, Venus, or any other planet.