Acousticproblemsoccurextensivelyinlongunderground spaces, for example, speech intelligibility problems in underground stations and noise pollution in road tunnels. Classical room acoustic theories cannot be used f~r long spaces since the assumption of a diffuse field is not satisfied when one dimension of a space is much greater than the other twv. To demonstrate the special properties of sound behaviour in long spaces and to provide guidelines for practical design, this paper outlines the results of Ph.D. research on acoustics of long spaces. In the research: (1) The basic characteristics of sound distribution and reverberation in long spaces have been theoretically studied; (2) The effectiveness of strategic architectural acoustic treatments in long spaces, especially for improving the intelligibility of multiple loudspeaker public address systems in underground stations, has been systematically tested in two 1:16 scale models; (3) The speech intelligibility in long spaces, particularly the differences in speech intelligibility among different languages, has been investigated by carrying out a series of articulation tests; and (4) Based on the above results, a model for predicting acoustic indices in long spaces has been developed.