The resonances of the Laplacian on perturbations of half-spaces of dimensio
ns greater than or equal to two, with either Dirichlet or Neumann boundary
conditions, are studied. An upper bound for the resonance counting function
is proven. If the domain has an elliptic, nondegenerate, nonglancing perio
dic billiard trajectory, it is shown that there exists a sequence of resona
nces that converge to the real axis.