SUMMARYIn this paper, we consider the Dirichlet and impedance boundary value problems for the Helmholtz equation in a non-locally perturbed half-plane. These boundary value problems arise in a study of timeharmonic acoustic scattering of an incident ÿeld by a sound-soft, inÿnite rough surface where the total ÿeld vanishes (the Dirichlet problem) or by an inÿnite, impedance rough surface where the total ÿeld satisÿes a homogeneous impedance condition (the impedance problem). We propose a new boundary integral equation formulation for the Dirichlet problem, utilizing a combined double-and single-layer potential and a Dirichlet half-plane Green's function. For the impedance problem we propose two boundary integral equation formulations, both using a half-plane impedance Green's function, the ÿrst derived from Green's representation theorem, and the second arising from seeking the solution as a single-layer potential. We show that all the integral equations proposed are uniquely solvable in the space of bounded and continuous functions for all wavenumbers. As an important corollary we prove that, for a variety of incident ÿelds including an incident plane wave, the impedance boundary value problem for the scattered ÿeld has a unique solution under certain constraints on the boundary impedance.