This chapter provides an introduction to air-sea gas exchange, covering the basic equations, models, experimental approaches, and a synthesis of current knowledge. The focus is on predicting gas exchange rates across the sea surface. The authors reference previous work and a book on gas transfer at water surfaces. They emphasize the importance of understanding gas transfer mechanisms, particularly for gases involved in geochemical cycles. The basic principles of air-sea gas exchange are outlined, including the flux equation F = κ(T)wΔC, where ΔC is the concentration difference driving the flux and κ(T)w is the total transfer velocity. The concentration difference is expressed as ΔC = CaH⁻¹ - Cw, where Ca and Cw are the gas concentrations in air and water, and H is Henry's Law constant. The total transfer velocity is broken down into components involving resistances in the air and water phases. The authors note that for many gases, either the air or water phase resistance is dominant, depending on their solubility and reactivity. For gases with high solubility (like CO₂), the water phase resistance is dominant, while for gases with low solubility (like O₂), the air phase resistance is dominant. The chapter focuses on evaluating the water phase transfer velocity (kw) for most gases of interest in geochemical cycles. The ΔC term is considered separately, as it varies by gas and is addressed in other chapters. The authors also mention that transfer velocities are determined through laboratory and field experiments, with a focus on wind tunnel and field measurements. The chapter aims to provide a synthesis of current knowledge to guide further research and understanding of air-sea gas exchange.This chapter provides an introduction to air-sea gas exchange, covering the basic equations, models, experimental approaches, and a synthesis of current knowledge. The focus is on predicting gas exchange rates across the sea surface. The authors reference previous work and a book on gas transfer at water surfaces. They emphasize the importance of understanding gas transfer mechanisms, particularly for gases involved in geochemical cycles. The basic principles of air-sea gas exchange are outlined, including the flux equation F = κ(T)wΔC, where ΔC is the concentration difference driving the flux and κ(T)w is the total transfer velocity. The concentration difference is expressed as ΔC = CaH⁻¹ - Cw, where Ca and Cw are the gas concentrations in air and water, and H is Henry's Law constant. The total transfer velocity is broken down into components involving resistances in the air and water phases. The authors note that for many gases, either the air or water phase resistance is dominant, depending on their solubility and reactivity. For gases with high solubility (like CO₂), the water phase resistance is dominant, while for gases with low solubility (like O₂), the air phase resistance is dominant. The chapter focuses on evaluating the water phase transfer velocity (kw) for most gases of interest in geochemical cycles. The ΔC term is considered separately, as it varies by gas and is addressed in other chapters. The authors also mention that transfer velocities are determined through laboratory and field experiments, with a focus on wind tunnel and field measurements. The chapter aims to provide a synthesis of current knowledge to guide further research and understanding of air-sea gas exchange.