Geospace Dynamics Constellation (GDC) is a mission concept to study the coupling between the magnetosphere and the ionosphere/thermosphere (IT) system, and how that coupled system responds to external energy input. GDC will address crucial scientific questions pertaining to the dynamic processes active in Earth’s upper atmosphere; their local, regional, and global structure; and their role in driving and modifying magnetospheric activity. GDC will be the first mission to address these questions on a global scale due to its use of a constellation of spacecraft that permit simultaneous multi-point observations. This investigation is central to understanding the basic physics and chemistry of the upper atmosphere and its interaction with Earth’s magnetosphere, but also will produce insights into space weather processes.