The concept of middle class is derived from sociology theories in the Western countries. Scholars in the Western countries developed this concept to explain the emergence of a social group in human society after the first industrial revolution. The emergence of middle class has exerted a profound impact on the political environment, labor market, and housing choices in a country or region, contributing to the urban social space reconstruction. The emergence of middle class also has resulted in gentrification, which has become an important phenomenon in a globalizing social-spatial context. Research on the middle class in Western countries has been closely associated with the changing concerns and paradigms in the literature. This paper reviews the research on the middle class in Western countries, which is divided into the following three periods: (1) the first period is before 1960. The research focused on the concept of the middle class, the spatial distribution of middle class communities, and middle class formation mechanism; (2) the second period is from the 1960s to the late 1980s. Studies focused on the middle class population and their choice of housing and gentrification; (3) the third period is since the 1990s. Subjects of studies are more diversified, focusing on redefining the middle class, reconsideration of the gentrification formation mechanism , the creative class, middle class communities and social spatial justice, consumer behavior, political role, and environmental awareness. The emergence of middle class in China is closely associated with the economic transition in China since the reform and the opening to the outside world in 1978. The establishment of market system, high-speed economic growth and urbanization, industrial restructuring, and the development of higher education have prompted the development of middle class in China. Studies done by researchers in China can be divided into two different stages: (1) before 2000, the research focused on introducing the concept and theories related to middle class in China; (2) after 2000, the middle class population has experienced a rapid increase in China, which has been attracting increasingly more attention from scholars. The research focuses on the definition of middle class in the context of the Chinese society, the formation mechanism of the middle class, and the gentrification in China. Compared with the research on middle class in Western countries, the study of middle class in China started later, and still does not address many relevant issues in sufficient details. There is a lack of systematic analysis of middle class in China, as well as unaddressed theoretical and methodological issues. Given the fact that middle class in China has been experiencing a dramatic growth, as was in other western countries a few decades ago, the research on middle class in China should better integrate western theories, and should also pay much more attention to the formation of middle class, the characteristics of middle class population and their living spaces, consumer behavior, community, gentrification, and stratification in the Chinese society.