Poisson Brackets with Prescribed Casimirs We consider the problem of constructing Poisson brackets on smooth manifolds $M$ with prescribed Casimir functions. If $M$ is of even dimension, we achieve our construction by considering a suitable almost symplectic structure on $M$, while, in the case where $M$ is of odd dimension, our objective is achieved by using a convenient almost cosymplectic structure. Several examples and applications are presented. Keywords:Poisson bracket, Casimir function, almost symplectic structure, almost cosymplectic structureCategories:53D17, 53D15