Crazing in thick polymer glass films and bulk glassy polymers is theoretically investigated. A simple microscopic model that treats an amorphous polymer at stresses above the yield stress as a viscous fluid with the scale-dependent surface tension is developed. Craze microstructural parameters including fibril diameter, spacing, and draw ratio are calculated. Crazing stress and critical thickness are determined as functions of material parameters and conditions of testing. The obtained results are in good quantitative agreement with experimental data.