A theoretical model has been developed to calculate the craze surface energy for monodisperse linear polymers based on the "tube model" of polymer melts. This model, which is a refinement of models previously proposed, assumes that the essential molecular processes of chain scission and disentanglement occur in the "active zone"-a region of thickness greater than or equal to the chain radius of gyration, situated at the interface between craze and bulk polymer. At high temperatures, greater than a critical temperature, T > T-tr, craze growth occurs by stress-assisted chain disentanglement; below a second critical temperature, T-cr, craze growth occurs by chain scission. Between these critical temperatures, which are molecular weight and strain-rate dependent, craze growth involves both scission and disentanglement. New experimental data on craze growth in monodisperse, linear polystyrene over a range of temperatures at two strain rates are presented, which are well described by the new theory. The theory describes all features of the data with only one fitting parameter, including small peaks observed in the crazing strain/temperature graphs in the mixed-mode region. Similar features have been reported recently in craze data from poly(methyl methacrylate) without explanation.