The Problem of P+ = 0 Mode in the Null-Plane Field Theory and Dirac's Method of Quantization

The null-plane quantization is studied with the emphasis on the P+ = 0 mode, by using Dirac's quantization for constrained systems. This mode is eliminated from the Hilbert space and the physical vacuum can be defined in a kinematical way. It enables us to construct the physical Fock space kinematically. Poincaré invariance is also studied in detail.