The thermal environment in outdoor spaces can significantly influence users’ thermal perception and thus their use of these spaces. Improving microclimatic conditions in urban spaces will most likely encourage people to spend more time outdoors, with the potential to improve their health and wellbeing, as well as boosting social cohesion. As well as enhancing the environmental quality of cities it should also eventually improve the quality of life of its citizens. This thesis is one of the first attempts to investigate the outdoor thermal comfort and the effect of cultural differences in hot arid climates. Case studies were carefully selected in two different parts of the world (Marrakech in North Africa and Phoenix-Arizona in North America) to represent a variety of users in similar climatic context. Field surveys, carried out during winter and summer, included: structured interviews with a standard questionnaire; observations of the human activities; and microclimatic monitoring. The results revealed that the solely physiological approach is insufficient to assess the outdoor thermal comfort conditions in hot arid climates. Environmental variables such as air temperature and solar radiation, could have a great impact on the use of the outdoor spaces in the hot arid climate, and may determine the number of people and activities in them. The study also shows that participants who usually spend more time outdoors due to their life style, “outdoors individuals”, tend to stay longer in the studied sites compared with the “indoors individuals” who spend more time indoors. This is probably because the “outdoor individuals” have better experience of the outdoor conditions and respective thermal conditions. Experience has a strong link with expectations so that according to their past experience, people prepare themselves for the expected weather by taking adaptive measures. People from different cultures in the hot arid climate are likely to evaluate their thermal conditions differently, have diverse thermal comfort requirements, and use urban public spaces differently as well. Further work needs to be done to cover more geographical areas within the hot arid climate. Such an expansion may generalise the findings of this study or explain any particularity associated with the sites of the current study. More research is also needed to investigate he thermal requirements and use of outdoor spaces by different social groups by using robust classification methods. Emphasis should be on investigating the influence of thermal comfort on the use of outdoor public spaces by young and older people, and how that may affect their health and will being in such climates.