The paper “Third-order theory for multi-directional irregular waves“, co-authored by Per Madsen and David Fuhrman, has appeared in the J Fluid Mechanics. The paper presents a new third-order solution for multi-directional irregular water waves in finite water depth. Expressions for the velocity potential at the free surface are provided, and the formulation incorporates the effect of an ambient current. Harmonic resonance may occur at third order for certain combinations of frequencies and wavenumber vectors, and in this situation the perturbation theory breaks down. Harmonic resonance is analysed for the case of a monochromatic short-crested wave interacting with a plane wave having a different frequency, and long-term simulations are presented using a high-order Boussinesq formulation in order to study the evolution of wave trains exposed to harmonic resonance. A link to the paper is here.