Many living organisms, including humans, possess an internal clock that regulates diverse life processes—such as gene expression and enzyme activity—in approximately 24-hour cycles known as circadian rhythms. Over the past century, research on biological clocks has made remarkable progress. In recent years, increasing attention has been directed toward emerging disciplines such as chrononutrition, chronopharmacology, and chronotoxicology, which emphasize the significance of timing in dietary intake, drug administration, and chemical exposure, respectively—all based on the intrinsic circadian rhythm of biological functions. Interest in circadian biology intensified following the awarding of the 2017 Nobel Prize in Physiology or Medicine to Dr. Jeffrey C. Hall and colleagues, who elucidated the molecular mechanisms governing circadian rhythms. This chapter provides an overview of the historical development of circadian research and introduces recent advances aimed at targeting the molecular machinery of the circadian clock to regulate rhythmic physiological functions.