RETOP is an open-source numerical tool developed in Matlab, designed to implement near-to-far-field transformations for light scattering or emission problems involving local inhomogeneities, such as scatterers, current sources, or combinations of both, embedded within a stratified medium. Quite uniquely, RETOP also calculates in-plane radiation diagrams for both guided and plasmonic modes. The far-field computations need the near-field (computed on a rectangular box that contains the inhomogeneities with any full-wave Maxwell solver). RETOP is especially relevant for calculating the scattering of nanoparticle on substrates. Special attention is made to the interface with COMSOL Multiphysics. In 2024, RETOP was integrated into COMSOL Multiphysics V6.2, see the application gallery “Far-Field Radiation With a Substrate” (excluding the option related to guided modes). RETOP is related to another freeware, RETICOLO [arXiv:2101.00901], developed by the same authors, which analyzes the diffraction by gratings using the Rigorous Coupled Wave Analysis (RCWA). The software is published in an archive journal: J. Yang, J.-P. Hugonin, and P. Lalanne, “Near-to-far field transformations for radiative and guided waves”, ACS Photonics 3, 395-402 (2016). Link to article.