We develop a general theory of symmetry reduction of states on (possibly non-commutative) * -algebras that are equipped with a Poisson bracket and a Hamiltonian action of a commutative Lie algebra g. The key idea advocated for in this article is that the "correct" notion of positivity on a * -algebra A is not necessarily the algebraic one whose positive elements are the sums of Hermitian squares a * a with a ∈ A, but can be a more general one that depends on the example at hand, like pointwise positivity on * -algebras of functions or positivity in a representation as operators. The notion of states (normalized positive Hermitian linear functionals) on A thus depends on this choice of positivity on A, and the notion of positivity on the reduced algebra A µ-red should be such that states on A µ-red are obtained as reductions of certain states on A. In the special case of the * -algebra of smooth functions on a Poisson manifold M , this reduction scheme reproduces the coisotropic reduction of M , where the reduced manifold M µ-red is just a geometric manifestation of the reduction of the evaluation functionals associated to certain points of M . However, we are mainly interested in applications to non-formal deformation quantization and therefore also discuss the reduction of the Weyl algebra, and of the polynomial algebra whose non-commutative analogs we plan to examine in future projects.