This chapter provides an introduction to air-sea gas exchange, covering the basic equations governing this process, models describing the gas transfer mechanism, experimental approaches, and a synthesis of wind tunnel and field results for predicting gas exchange rates. The authors reference their previous work and a book on gas transfer at water surfaces, highlighting recent developments in the field. The fundamental equations for air-sea gas exchange are detailed, including the concentration difference and total transfer velocity. The chapter focuses on evaluating the water phase transfer velocity ($k_w$) for gases of interest in geochemical cycling, as most gases have high Henry's Law Constants ($H$) and low partitioning into the air phase. The chapter also discusses the dominance of water phase resistance in many cases and the importance of understanding individual gas-specific concentration differences.This chapter provides an introduction to air-sea gas exchange, covering the basic equations governing this process, models describing the gas transfer mechanism, experimental approaches, and a synthesis of wind tunnel and field results for predicting gas exchange rates. The authors reference their previous work and a book on gas transfer at water surfaces, highlighting recent developments in the field. The fundamental equations for air-sea gas exchange are detailed, including the concentration difference and total transfer velocity. The chapter focuses on evaluating the water phase transfer velocity ($k_w$) for gases of interest in geochemical cycling, as most gases have high Henry's Law Constants ($H$) and low partitioning into the air phase. The chapter also discusses the dominance of water phase resistance in many cases and the importance of understanding individual gas-specific concentration differences.