This chapter explores oxygen demand as a fundamental measure of organic matter in aquatic systems, with emphasis on biochemical oxygen demand (BOD), chemical oxygen demand (COD), and theoretical oxygen demand (ThOD). These parameters provide critical insights into the extent of biodegradable versus non-biodegradable pollutants and the potential impact of discharges on dissolved oxygen levels. Detailed discussion is given on test methods, reaction principles, and comparative interpretation of BOD and COD, along with their role in assessing treatment efficiency and regulatory compliance. The chapter also highlights the application of stoichiometric oxygen demand (ThOD) as a predictive tool, linking chemical composition of pollutants with expected oxygen depletion. Together, these metrics form the quantitative foundation for water quality monitoring and design of wastewater treatment systems.