At the beginning of the third millennium, urban public spaces as third places are focal points of urban design activities. They play an important role in social interactions. Public spaces belong to all citizens and they are not only defined by physical aspects but also by human activities.Public spaces have a long history in urban development process and structures of the most cities have been formed around them. After industrial revolution, urban public spaces lost their main function because of emerging of automobile and so new concepts propounded. Since three decades ago, as a result of developing ICT and increasing public interests, quality issues in urban spaces have been considered so much more than before. These factors have led to diverse theoretical trends in related to urban public spaces by different theorists.The aim of this paper is reviewing viewpoints of the most important experts related to urban public spaces and categorizing them. For this purpose, a documentary research method through valid resources and a comparative analysis process have been applied.In this order, after studying the last concepts, theorists related to public spaces have been divided into six topics: environmental-sustainability, spatial-visual cognition, social interactions, pedestrian-orientation, security and human scale, and environmental-behavioral effects. While a theorist may have discussed about the subject from different aspects, but in this research, he (she) has been classified based on his (her) most predominant trend. Accordingly, Serge Chermayeff, Graham Haughton, Colin Hunter, Hugh Barton and Richard Rogers in environmental-sustainability trend, Sitte, Cullen, Rob Krier, Rossi, Christopher Alexander, Cliff Moughtin, Ali Madanipour and Jahanshah Pakzad in spatial-visual cognition trend, Arendt, Zucker, Jane Jacobs, William Whyte, Jan Gehl, Oldenburg and C.C. Marcus in social interactions trends, Garnier, Speiregen, Halprin, Bacon, Bill Hillier and Michael E.Arth in pedestrian orientation trend, Mammford, Francis Tibbalds, Andres Duany, Plater-Zyberk, Peter Katz, Zeninca and Bernan in security and human scale trend, Lynch, Rapoport, Romedi Passini, More, Jon Lang and Hossein Bahreini in environmental-behavioral effects trend have been classified.The results show that there has been diverse and variable trends related to urban public spaces specially in the last century. So, the most significant contemporary related theories can be categorized based on their dominant trend into three main periods: from late 19th century to 1960, from 1960 to 1990 and from 1990 until now.In the first period, theorists have been emphasized on spatial and visual cognition in public spaces. So, concepts such as artistic principles of city planning, townscape, serial vision and architecture of the city were dominated. While in the second period, the focus has been on environmental- behavioral effects, strengthening social interactions and developing pedestrianorientation in urban spaces. Theories like third place, monumental plaza, way finding, city image, space syntax and new pedestrianism are the most important concepts in this period.Finally, in the last period, security-oriented and human-scale designing of public realms in addition to environmental considerations and sustainability issues have become more important.Sustainable city and new urbanism are the main theories in this recent period.