  This paper is devoted to the investigation of the boundary regularity for the
Poisson equation $${{cc} -\Delta u = f & \text{in} \Omega u= 0 & \text{on}
\partial \Omega$$ where $f$ belongs to some $L^p(\Omega)$ and $\Omega$ is a
Reifenberg-flat domain of $\mathbb R^n.$ More precisely, we prove that given an
exponent $\alpha\in (0,1)$, there exists an $\varepsilon>0$ such that the
solution $u$ to the previous system is locally H\"older continuous provided
that $\Omega$ is $(\varepsilon,r_0)$-Reifenberg-flat. The proof is based on
Alt-Caffarelli-Friedman's monotonicity formula and Morrey-Campanato theorem.
