This is a conversation with Brian Toon about the NASA TC4 project. TC4 stands for "Tropical Composition, Cloud and Climate Coupling" and deals with the chemical, dynamic, and physical processes occurring in the tropical upper troposphere and tropopause transitional layer. In our conversation we cover some of the scientific goals of the project, the tools and technologies used (aircraft and satellites) as well as the general project organization. A later episode will cover the project from the perspective of an ER-2 pilot.