This paper reviews the various effects of an audience on the acoustical properties of an auditorium. At the most basic level, the total sound absorption of both occupied and unoccupied chairs can be predicted in terms of the perimeter/area ratio of audience seating blocks. Using this technique, reverberation chamber measurements of occupied chairs can be used to estimate the reverberation times of fully occupied halls. This technique has been extended to provide estimates of typical sound-absorbing properties of occupied chairs for all sizes of seating blocks. Partially occupied seating areas present further problems. Audience effects on early- and late-arriving sound levels and early/late sound ratios are related to the geometry of the halls as well as to changes in the total sound absorption. Interaural cross correlations and lateral energy fractions are not greatly influenced by the presence of an audience. Changes to the more detailed spectra of the direct and early arriving sound show that the effect of the audience on the seat dip attenuation can vary with the type of seating as well as other properties of the hall.