This article presents a systematic introduction to theoretical and experimental aspects of photochemistry, along with typical examples and applications. It gives a survey of photochemical laws and fundamental principles of photophysics and photochemistry, including multiphoton processes that occur at high light intensity laser irradiation. Descriptions of experimental techniques are given for both continuous irradiation and flash photolysis, including laser spectroscopy and femtochemistry. Detailed discussions of the important theories and representative reactions in the photochemistry of organic, inorganic, and coordination compounds are also presented. Finally, some of the more significant roles photochemistry plays in nature and in modern life, such as photosynthesis, information storage, and phototherapy, are briefly introduced.