Albeit there is a general consensus that urban design in the twentieth century failed in relation to space use, systematic research on this aspect has been very limited or at most inadequate in the developing countries. This study focuses on the above failure of urban design in this century with specific reference to the developing countries through an analysis of Dar es Salaam city centre, Tanzania. The modern city centre was analyzed critically through observation, document review, analysis of maps and plans, coupled with interview. The aim is to add to the understanding about: why and how urban design in this century failed with respect to space use in the developing countries, and how urban design in these countries could be made more successful in this respect. The results of the study suggest that dissociation of urban design in this century from prevailing economic, technological, and climatic conditions in the developing countries has been the overriding factor for its poor performance with respect to space use in these countries. The results show that because urban design in this century detached itself from prevailing economic, technological, and climatic conditions in the developing countries, it lead to urban spatial forms which are deterrent to meaningful space use. The results also provide important clues about how urban design in the developing countries could be improved with respect to space use within their existing economic, technological, and climatic circumstances. Particularly, the study reveals that through mixed land use patterns, traffic circulation systems which give less priority to vehicular traffic, less emphasis on urban solids, reconsideration of plot ratios, and more appropriate urban space treatment, urban design in the developing countries could be more successful in relation to space use.