This article reviews the evolving theories around the causes of dental caries over the last 120 years. It discusses how early theories focused on deficiencies in diet, but it is now understood that bacteria like Streptococcus mutans play a key role by metabolizing sugars like sucrose in plaque and lowering the pH. While S. mutans was long thought to be the primary cause, more recent studies find a broader range of bacteria are involved. In addition to sugars, starches can also cause caries. Future caries prevention strategies may focus more broadly on targeting plaque acidity and acidogenic microbes, rather than only S. mutans. An optimal diet remains important to reducing caries risk.