This lecture covers the reactivity of organic matter in water systems, including the different pools and ageing processes of organic matter, the operational definitions of carbon, oxygen, hydrogen, and nitrogen, as well as the transformation and roles of organic matter in the environment. It also discusses the hydrophobicity and hydrophilicity of organic matter, the complexation sites on organic matter, and the measure of organic matter hydrophobicity using Kow. The lecture concludes with a discussion on the credits related to the study of organic matter reactivity.