The combined PROSPECT leaf optical properties model and SAIL canopy bidirectional reflectance model, i.e. PROSAIL, has been used for about fifteen years to increase our understanding of plant canopy spectral and bidirectional reflectance in the solar domain and to develop new methods of vegetation biophysical properties retrieval. It links the spectral variation of canopy reflectance with its directional variation. This link is the key to simultaneously estimate biophysical/structural canopy variables for applications in agriculture, plant physiology, and forestry at different scales. PROSPECT and SAIL are still evolving: they have undergone recent improvements both at the leaf and the plant levels and became one of the most popular radiative transfer tools in these domains due to their ease of use, their robustness, and because they have been validated by many lab/fleld/space experiments over the years. This paper is intended to review this subject, which has been extensively researched in optical remote sensing.