We show that the Porous Medium Equation and the Fast Diffusion Equation, \dot u - \Delta {u^m} = f with m\in (0, \infty ), can be modeled as a gradient system in the Hilbert space H^{-1}(\Omega ), and we obtain existence and uniqueness of solutions in this framework. We deal with bounded and certain unbounded open sets \Omega \subset \mathbb{R}^{n} and do not require any boundary regularity. Moreover, the approach is used to discuss the asymptotic behaviour and order preservation of solutions., Samuel Littig, Jürgen Voigt., and Obsahuje seznam literatury