The structure of a cross-linked polymer network depends not only on its chemical constituents but also on the conditions under which the gel is polymerized. Highly nonuniform spatial distributions of polymer network concentration and cross-linking density are observed in most polymer gels. The inhomogeneities, as studied using light scattering, are shown to result from two origins : one from the dynamic critical fluctuations of the polymer solution at the onset of gelation, and the other from the domain formation due to the microphase separation. Both are directly related to the phase equilibrium properties of the gel during the gelation process. Those fluctuations of polymer density are frozen in the gel structure permanently. In addition to these permanent spatial fluctuations, a polymer network undergoes thermal dynamical concentration fluctuations which diverge at the critical point. These three types of fluctuations, two static and one dynamic, account for the nature of gel inhomogeneities. The static spatial fluctuations reversibly increase and diverge at the spinodal line, although they are permanent inhomogeneities. A qualitative interpretation is given to account for this phenomenon.