Preliminaries -- Scales, Tools and Reminiscences -- Observing Clouds and Their Optical Properties -- Fundamentals -- A Primer in 3D Radiative Transfer -- Numerical Methods -- Approximation Methods in Atmospheric 3D Radiative Transfer Part 1: Resolved Variability and Phenomenology -- Climate -- Approximation Methods in Atmospheric 3D Radiative Transfer Part 2: Unresolved Variability and Climate Applications -- 3D Radiative Transfer in Stochastic Media -- Effective Cloud Properties for Large-Scale Models -- Broadband Irradiances and Heating Rates for Cloudy Atmospheres -- Longwave Radiative Transfer in Inhomogeneous Cloud Layers -- Remote Sensing -- 3D Radiative Transfer in Satellite Remote Sensing of Cloud Properties -- Horizontal Fluxes and Radiative Smoothing -- Photon Paths and Cloud Heterogeneity: An Observational Strategy to Assess Effects of 3D Geometry on Radiative Transfer -- 3D Radiative Transfer in Vegetation Canopies and Cloud-Vegetation Interaction.