The photochemistry of Titan's atmosphere has been studied using updated chemical models and rate coefficients. The model incorporates exospheric boundary conditions, vertical transport, and condensation processes at the tropopause. Five major processes control Titan's atmospheric composition and evolution: photolysis of methane, hydrogen escape, synthesis of hydrocarbons, nitrogen-hydrocarbon coupling, and oxygen-hydrocarbon coupling. Starting with nitrogen, methane, and water, the model accounts for observed concentrations of minor species by the Voyager IRIS and UVS instruments. Photochemistry converts simple atmospheric species into complex organic compounds, which condense at the tropopause and deposit on the surface. Titan may have lost significant amounts of methane, nitrogen, and carbon monoxide over geological time. Abiotic organic synthesis on Titan may have implications for the origin of life on Earth. Titan's atmosphere is reducing, with an oxidation state between that of terrestrial planets and Jovian planets. The abundance of organic compounds in the stratosphere indicates the type of chemistry prevailing on Titan. The model suggests that most observed compounds are derived from a few parent molecules via photochemical processes. The atmosphere of Titan is conceptually simple, similar to Mars, where photochemistry explains its present composition and past evolution. The model assumes the presence of primitive parent molecules nitrogen, methane, and water. The origin of nitrogen and methane can be traced to Titan's volatile endowment at formation. Water is continuously replenished by meteoritic sources. Interaction with ultraviolet radiation, energetic electrons, and cosmic rays produces reactive radicals, which form more complex molecules. The model aims to explain the abundances of minor constituents listed in Table 1A. However, the task is hindered by lack of laboratory kinetics data and uncertainties in vertical mixing parameters. The model is a preliminary attempt to integrate observations and chemical kinetics data. The photochemical model is somewhat elaborate but conceptually simple. It can be understood in terms of a few chemical cycles or schemes, similar to those in terrestrial atmospheres. The atmosphere is divided into two regions: thermosphere/mesosphere and stratosphere. The thermosphere/mesosphere involves dissociation of nitrogen, water, and methane by direct photolysis or electron impact, leading to the production of important species like HCN, CO, CO2, C2H2, C2H4, and CH3C2H. These species can act as precursors for stratospheric chemistry by catalyzing the dissociation of methane. Photosensitized processes in the stratosphere are responsible for producing C2H6 and other compounds. Stratospheric chemistry can also account for the presence of C4H2, HC3N, and C2N2. The primary processes in the thermosphere/mesosphere are well-defined, while the secondary processes in the stratosphere are less so. The first important problem is assessing the importance of stratospheric chemistry relative to mesospheric chemistry. The second importantThe photochemistry of Titan's atmosphere has been studied using updated chemical models and rate coefficients. The model incorporates exospheric boundary conditions, vertical transport, and condensation processes at the tropopause. Five major processes control Titan's atmospheric composition and evolution: photolysis of methane, hydrogen escape, synthesis of hydrocarbons, nitrogen-hydrocarbon coupling, and oxygen-hydrocarbon coupling. Starting with nitrogen, methane, and water, the model accounts for observed concentrations of minor species by the Voyager IRIS and UVS instruments. Photochemistry converts simple atmospheric species into complex organic compounds, which condense at the tropopause and deposit on the surface. Titan may have lost significant amounts of methane, nitrogen, and carbon monoxide over geological time. Abiotic organic synthesis on Titan may have implications for the origin of life on Earth. Titan's atmosphere is reducing, with an oxidation state between that of terrestrial planets and Jovian planets. The abundance of organic compounds in the stratosphere indicates the type of chemistry prevailing on Titan. The model suggests that most observed compounds are derived from a few parent molecules via photochemical processes. The atmosphere of Titan is conceptually simple, similar to Mars, where photochemistry explains its present composition and past evolution. The model assumes the presence of primitive parent molecules nitrogen, methane, and water. The origin of nitrogen and methane can be traced to Titan's volatile endowment at formation. Water is continuously replenished by meteoritic sources. Interaction with ultraviolet radiation, energetic electrons, and cosmic rays produces reactive radicals, which form more complex molecules. The model aims to explain the abundances of minor constituents listed in Table 1A. However, the task is hindered by lack of laboratory kinetics data and uncertainties in vertical mixing parameters. The model is a preliminary attempt to integrate observations and chemical kinetics data. The photochemical model is somewhat elaborate but conceptually simple. It can be understood in terms of a few chemical cycles or schemes, similar to those in terrestrial atmospheres. The atmosphere is divided into two regions: thermosphere/mesosphere and stratosphere. The thermosphere/mesosphere involves dissociation of nitrogen, water, and methane by direct photolysis or electron impact, leading to the production of important species like HCN, CO, CO2, C2H2, C2H4, and CH3C2H. These species can act as precursors for stratospheric chemistry by catalyzing the dissociation of methane. Photosensitized processes in the stratosphere are responsible for producing C2H6 and other compounds. Stratospheric chemistry can also account for the presence of C4H2, HC3N, and C2N2. The primary processes in the thermosphere/mesosphere are well-defined, while the secondary processes in the stratosphere are less so. The first important problem is assessing the importance of stratospheric chemistry relative to mesospheric chemistry. The second important