Thin film experiments have shown that glassy thermoplastics exhibit enhanced molecular mobility across a 10 nm surface layer. This study examines its relevance to crazing. Surface mobility produces a steep yield stress gradient, which constrains the growth of plastic zones from surface flaws. During tensile tests, stresses in these zones increase until the bulk specimen either yields or fractures. Polycarbonate can sustain rising local stresses and strains without damage because its chains have relatively small cross-sectional areas, but high stresses in polystyrene produce chain scission, accelerated relaxation and cohesive failure. The partially degraded plastic zone breaks down to form craze fibrils, an internal necking process that is repeated during craze propagation and fibril drawing. In combination with the linear elastic fracture mechanics model for craze initiation, constraints on yielding around symmetrical surface flaws account for the strong dependence of critical tensile crazing stress sigma(1cz) on the second principal stress sigma(2cz) in biaxial tests. (C) 2012 Elsevier Ltd. All rights reserved.