We present a method of calculating one-loop effective potentials for quantum fluctuations from quasinormal modes. We will demonstrate the method for a harmonic oscillator and a scalar in De Sitter space. After briefly discussing its application in Anti-de Sitter space, we will speculate on the physical meaning of this mathematical trick as well as its extension to fields with higher spin, to product spaces, and to spaces with non-Dirichlet boundary conditions.