In many Lagrangian field theories one has a Poisson bracket defined on the
space of local functionals. We find necessary and sufficient conditions for a
transformation on the space of local functionals to be canonical in three
different cases. These three cases depend on the specific dimensions of the
vector bundle of the theory and the associated Hamiltonian differential
operator. We also show how a canonical transformation transforms a Hamiltonian
evolutionary system and its conservation laws. Finally we illustrate these
ideas with three examples.