This paper considers the dynamic response of floors to crowd loads including the influence of crowd size and the co-ordination of the walkers. Experiments were undertaken on two concrete floors utilising groups of up to 32 people walking across the floors at different pacing rates. In each case there was a definite trend for the peak accelerations to increase with increasing group size. However, the increase is not linear and the maximum accelerations were no more than double those generated by an individual. The paper also considers the calculation of floor response. It extends a previously published method for determining the acceleration generated by an individual walking to cover groups. It evaluates response in terms of Vibration Dose Values for use in serviceability assessments to BS 6472. B. R. Ellis, BSc, PhD, CEng, MIStructE BRE Centre for Structural Engineering