Forming urban heat islands is a serious challenge of the present age and a concern of academic communities. Attention to this issue has increased tremendously in scientific articles and researches, particularly in the last decade. With regard to the importance of the issue, the aim of this study is to form a systematic review and thematic analysis of articles and other researches in this field. Moreover, the content of the key articles was analyzed and the most important factors affecting the formation of urban heat islands were represented in a diagram and each of them was analyzed separately. As examined, urban heat islands are affected by climatic and urban construction factors. The climatic factors include sunlight, wind speed and direction, cloud cover, soil moisture, air humidity, precipitation, latitude, seasonal topographic variations, and the proximity to the river and the sea. Although these factors are almost uncontrollable in the existing cities, they have enormous importance for deciding new towns location and making the decision for city development direction. The second group of factors mainly related to the design of cities is controllable. Among these factors, the most important one is the amount and quality of vegetation in the city. Other factors such as land use, urban density, type of materials used on the facade of the buildings and streets, form and geometry of the city, and also transportation mode could influence the intensity and extent of urban heat islands.