Abstact: In this thesis we study the Casimir effect from the point of veiw of constrainted systems. The common point between the two issus is "the boundry conditions". We consider boundary conditions as Dirac constraints in the process of quantization of Klein-Gordon, electrodynamics and bosonic string theories. For this reason, we find infinit chains of constraints at the boundaries, and impose them on the Furrier modes of the fields. In this way we reach to the reduced phase space. We then quantize the system by inverting the Dirac brackets to commutators and express the energy operator in terms of physical modes. The Casimir effect then emerges as a result of comparission of vacuum modes in the energy operator with the remaining modes when some boundary conditions are present. Hence we show that the Casimir force is directly a consequence of imposing the Dirac constraints. Key Words: Casimir effect,constrainted System,boundary conditions,Dirac poissons.