This study reviews world literature on the strength-related effect of duration and rate of loading on wood and wood-based materials. Also, early developments are discussed which led to the currently used permanent loading factor of 9/16. Published data on duration of load are reanalyzed and compared. The comparisons suggest that the effect of stress level on duration of stress for wood is greater for shear than for bending or compression. The comparisons also suggest that the stress level effect is greater in hardboard, particleboard, and plywood than in solid wood.