The first 120 years of chiropractic included distinct waves of ideas. The earliest writings on chiropractic comprised the first wave. Each subsequent wave is characterized by distinct sets of theories. The political, social, cultural, economic, and legal forces helped to define each period. Personal worldviews and various forms of clinical and empirical research dictated the form of the ideas. The most characteristic distinction for each wave is that the books and articles published during that period references the literature from the previous wave.