In the Western medicine system, scholars have explained individual differences in terms of behaviour and thinking, leading to the emergence of various classification theories on individual differences. Traditional Chinese medicine has long observed human constitutions. Modern Chinese medicine studies have also involved study of human constitutions; however, differences exist in the ways traditional and modern Chinese medicine explore individual constitutions. In the late 1970s, the constitutional theory of Chinese medicine was proposed. This theory takes a global and dynamic view of human differences (e.g., the shape of the human body, function, psychology, and other characteristics) based on arguments from traditional Chinese medicine. The establishment of a standard for classifying constitutions into nine modules was critical for clinical application of this theory. In this review, we describe the history and recent research progress of this theory, and compare it with related studies in the western medicine system. Several research methods, including philology, informatics, epidemiology, and molecular biology, in classifying constitutions used in the constitutional theory of Chinese medicine were discussed. In summary, this constitutional theory of Chinese medicine can be used in clinical practice and would contribute to health control of patients.