The paper “Phase dynamics of periodic waves leading to the KP equation in 3+1 dimensions“, co-authored by Daniel Ratliff and Tom Bridges, has been accepted for publication in the Proceedings of the Royal Society of London. In the paper it shown that the KP equation is the relevant modulation equation for bifurcation from periodic travelling waves when the wave action flux has a critical point. Moreover, the emergent KP equation arises in a universal form, with the coefficients determined by the components of the conservation of wave action. The theory extends to any space dimension and time, but the emphasis in the paper is on the case of 3+1. Motivated by light bullets and quantum vortex dynamics, the theory is illustrated by showing how defocussing NLS in 3+1 bifurcates to KP in 3+1 at criticality.  The final-form preprint is available here.