This book provides students with the tools necessary to understand the processes that control the chemical species present in waters of both natural and engineered systems. After providing basic information about water itself and the chemical composition of water in environmental systems, the text covers the necessary theory (thermodynamics, activity, and kinetics) and background material to solve problems. It emphasizes that both equilibrium and kinetic processes are important in aquatic systems. The book does not merely focus on inorganic constituents, but also on the fate and reactions of organic chemicals. The solving of quantitative equilibrium and kinetic problems using mathematical, graphical, and computational tools is emphasized throughout presentations on acid-base chemistry, complexation of metal ions, solubility of minerals, and oxidation-reduction reactions. The use of these problem-solving tools is then extended in the presentation of topics relevant to natural systems, including dissolved oxygen, nutrient chemistry, geochemical controls on chemical composition, photochemistry, and natural organic matter. The kinetics and equilibria relevant to engineered systems (e.g., chlorination and disinfection chemistry, sorption and surface chemistry) and organic contaminant chemistry are also discussed. Numerous in-chapter examples that show the application of theory and demonstrate how problems are solved using algebraic, graphical, and computer-based techniques are included. Examples are relevant to both natural waters and engineered systems.