Monomeric resins are frequently polymerized at a temperature that is far below the desired glass transition temperature for the final polymer network. This work quantitatively investigates the relationship between the glass transition temperature (T-g), the cure temperature (T-cure), the maximum achievable glass transition temperature for a particular monomer formulation (T-gmax), and the network heterogeneity as indicated by the half-width of the glass transition region (T-g1/2width) for both chain growth and step growth polymerization systems. Various monomer resins were systematically photopolymerized at 25, 50, 75, and 100 degrees C to the maximum achievable double bond conversion with living radical photopolymerizations employed as necessary to eliminate issues associated with postcuring during heating of the samples for dynamic mechanical analysis. For polymer samples that have the potential for much higher glass transition temperatures than the curing temperature (i.e., where T-cure < T-gmax -T-g1/2width), mass transfer limitations and the network heterogeneity combine to control T-g such that T-g approximate to T-cure + T-g1/2width. For systems polymerized at temperatures closer to their maximum glass transition temperature, i.e., where T-cure >= T-gmax -T-g1/2width, the polymer structure becomes the dominant factor and T-g approximate to T-gmax. These relationships were broadly and successfully applied to cross-linked networks formed from both chain growth and step growth polymerizations, incorporating networks of vastly different heterogeneity, with T-g's ranging from 35 to 205 degrees C with approximately 10% error found between predicted and experimental glass transition temperatures.