librat is a flexible, modular radiative transfer model designed to simulate the light environment (reflectance, lidar return) of structurally and spectrally realistic 3D scenes. librat is specifically designed for simulating remote sensing instruments and observations, at arbitrary scales from cm to km. librat has many features specific to these aims, including a wide range of options for dealing with sensor and illumination characteristics etc., including for time-resolved lidar signals. librat is designed with scientific applications in mind, and as a result maintains as much information as possible about scattering properties, with interaction order (to 10s or even 100s of interactions) and wavelength, in order to maintain fidelity of scattering information. It is not designed for rapid rendering e.g. for realistic scene visualisation and animation; there are many tools which make use of hardware acceleration and approximations which are more suited to that. librat has been used for a wide range of remote sensing applications including sensor modelling, biophysical parameter retrieval, algorithm development and testing, and model benchmarking.