The NASA TC4 (Tropical Composition, Cloud and Climate Coupling) mission investigated the structure, properties and processes in the tropical Eastern Pacific. A-train satellite observations provided crucial information on the spatial and temporal variations of this region; however, carefully planned TC4 aircraft observations were required both to validate satellite data and to provide critical observations not available from the satellites. High altitude aircraft collected tropopause data while the medium altitude aircraft provided profiles and structure measurements of the tropical upper troposphere and lower stratosphere.