Four types of gels are distinguished according to their structure: 1, ordered, lamellar gels (e.g., clays, soaps, and similar mesophases); 2, covalent polymer networks; 3, polymer networks formed by physical aggregation of polymer chains through the agency of multistranded helices or of crystalline domains; 4, particulate disordered gels (e.g., fibrillar or reticular precipitates, aggregated globular proteins, etc.). Continuity of structure over macroscopic dimensions and permanency of that structure are cited as the underlying features common to gels in general. The preponderance of the gels and gelling processes brought before this Discussion are of types 2 or 3. The statistical theory of molecular distributions in randomly interlinked systems is reviewed and the theory of gelation is discussed in relation to experimental measurements and observations.