Boundary Behavior of Solutions of the Helmholtz Equation This paper is concerned with the boundary behavior of solutions of the Helmholtz equation in $\mathbb{R}^\di$. In particular, we give a Littlewood-type theorem to show that the approach region introduced by Kor\'anyi and Taylor (1983) is best possible. Keywords:boundary behavior, Helmholtz equationCategories:31B25, 35J05