Some new results are presented on the theory of Hamiltonian systems with first-class constraints. In these systems it is possible to separate the physical part from the gauge part by transforming to canonical coordinates in which the constraints are a subset of the new momenta; this construction is accomplished by algebraic methods and the use of a set of Hamilton-Jacobi-like equations. Finally, the problem and meaning of evolution in systems with weakly vanishing Hamiltonian is commented on.