A large number of researchers are working in the area of nonlinear optics, which is the study of all effects that can be described as multi-photon interactions in various materials systems, including cases where the frequency of one or more photons tends to zero. Motivated by the needs of these researchers, meetings have sprung up over the past few years under the name “Foundations of Nonlinear Optics.” The two most recent of these meetings took place at Lehigh University in 2015, and at Tufts University in 2016, and the next one will take place at the University of Bahamas.