This review summarizes the current understanding of Titan's atmospheric composition and the chemistry that leads to its observed composition. It begins with an overview of Titan's atmospheric composition, derived from measurements by the Cassini-Huygens mission, the Atacama Large Millimeter/submillimeter Array, and other ground- and space-based telescopes. The review focuses on the typical vertical profiles of gases at low latitudes rather than global and temporal variations. The main body of the review presents a chemical description of how complex molecules are believed to arise from simpler species, considering all known "stable" molecules—those that have been uniquely identified in the neutral atmosphere. The last section of the review is devoted to the gaps in our present knowledge of Titan's chemical composition and how further work may fill those gaps. Titan, Saturn's largest moon, has a dense atmosphere composed primarily of molecular nitrogen and methane, making it the only such body in the Solar System. This atmosphere is largely anoxic, with little oxygen to terminate complex organic reactions, resulting in a chemical wonderland with a wide array of complex organic molecules. The review discusses the photochemical processes in Titan's atmosphere, including photodissociation, ionization, and radical reactions, which lead to the formation of complex molecules and haze particles. The review also addresses the vertical profiles of gases in Titan's atmosphere, noting that methane is more abundant in the troposphere than above it due to the "cold trap" effect. The atmosphere's temperature structure is influenced by various processes, including convection, solar heating, and radiative cooling. The review highlights the importance of understanding Titan's atmosphere for astrobiology, as it provides insights into the potential for life in the Solar System and beyond. The review concludes with a discussion of future research directions in Titan atmospheric composition studies.This review summarizes the current understanding of Titan's atmospheric composition and the chemistry that leads to its observed composition. It begins with an overview of Titan's atmospheric composition, derived from measurements by the Cassini-Huygens mission, the Atacama Large Millimeter/submillimeter Array, and other ground- and space-based telescopes. The review focuses on the typical vertical profiles of gases at low latitudes rather than global and temporal variations. The main body of the review presents a chemical description of how complex molecules are believed to arise from simpler species, considering all known "stable" molecules—those that have been uniquely identified in the neutral atmosphere. The last section of the review is devoted to the gaps in our present knowledge of Titan's chemical composition and how further work may fill those gaps. Titan, Saturn's largest moon, has a dense atmosphere composed primarily of molecular nitrogen and methane, making it the only such body in the Solar System. This atmosphere is largely anoxic, with little oxygen to terminate complex organic reactions, resulting in a chemical wonderland with a wide array of complex organic molecules. The review discusses the photochemical processes in Titan's atmosphere, including photodissociation, ionization, and radical reactions, which lead to the formation of complex molecules and haze particles. The review also addresses the vertical profiles of gases in Titan's atmosphere, noting that methane is more abundant in the troposphere than above it due to the "cold trap" effect. The atmosphere's temperature structure is influenced by various processes, including convection, solar heating, and radiative cooling. The review highlights the importance of understanding Titan's atmosphere for astrobiology, as it provides insights into the potential for life in the Solar System and beyond. The review concludes with a discussion of future research directions in Titan atmospheric composition studies.