  In this article I summarize the current state of knowledge about the
composition of Titan's atmosphere, and our current understanding of the
suggested chemistry that leads to that observed composition. I begin with our
present knowledge of the atmospheric composition, garnered from a variety of
measurements including Cassini-Huygens, the Atacama Large
Millimeter/submillimeter Array (ALMA), and other ground and space-based
telescopes. This review focuses on the typical vertical profiles of gases at
low latitudes, rather than global and temporal variations. The main body of the
paper 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 paper is devoted to the gaps in our present knowledge of Titan's
chemical composition and how further work may fill those gaps.
