Exact Results in Quantum Theory

In this talk, I will introduce an Exact Renormalization Group proposed by Polchinski in Nucl. Phys. B231 (1984) 269. Its principles are very general and should apply to any system whose symmetries are preserved by a momentum-space cutoff. As an example, I will consider a four-dimensional \phi^4 theory with a momentum cutoff. After identifying relevant and irrelevant operators, the exact renormalization group equations describing the cutoff dependence of the effective Lagrangian will be derived.