Systems to measure gas production to study digestion kinetics have been developed at several locations. The system developed at Cornell University and the rationale behind its evolution are described with an emphasis on whether venting after each observation is necessary and on choice of sensors. Different non-linear-models used to fit gas production data are discussed with an emphasis on the dual-pool logistic model. The third section of the paper includes a theoretical discussion on how gas data can be integrated with data on passage to predict ruminal digestibility. The final section addresses the practical applications of these gas data and ways in which they can be used in models like the Cornell net carbohydrate and protein system. Also included are evaluations of ensiled and freeze-dried samples from the same source as an indication of how gas systems can be used to evaluate the soluble fractions of forages.