In this article we show that boundary conditions call be treated as Lagrang
ian and Hamiltonian constraints, Using the Dirac method: we find that bound
ary conditions are equivalent tu all infinite chain of second class constra
ints. which is a new feature ill the context of constrained systems. Constr
ucting the Dirac brackets and the reduced phase space structure for differe
nt boundary conditions, we show why mode expanding and then quantizing a fi
eld theory with boundary conditions is the proper wag. We also show that il
l a quantized field theory subjected to the mixed boundary conditions, the
field components are non-commutative.