Several wind energy concepts utiliz e airborne systems that contain lighterthan-air gas, which supplements aerodynamic lift and expands these systems' available operating regimes. While lighter-than-air systems can incorporate the traction and crosswind fl ight motions of their heavier-than-air counterparts, several lighterthan-air concepts have also been designed to deliver large amounts of power under completely stationary operation and remain aloft during periods of intermittent wind. This chapter provides an overview of the history of LTA airborne wind energy concepts, including the design drivers and principal design constraints. The focus then turns to the structural and aerodynamic design principles behind lighter than air systems, along with fundamental fl ight dynamic principles that must be addressed. A prototype design developed by Altaeros Energies is examined as an example of the application of these principles. The chapter closes with suggestions for future research to enable commercially-viable LTA systems. Why Lighter-Than-Air?Lighter-than-air (LTA) technology refers to airborne systems that use helium, hydrogen, or other sources of buoyancy to provide lift. LTA systems include freefl ying blimps, airships, dirigibles, and z eppelins; as well as stationary, tethered systems such as moored balloons or aerostats. For decades, hundreds of large tethered aerostats have been deployed to lift heavy pieces of equipment into the air for long periods of time. Aerostats are the only tethered aeronautical platforms that have consistently demonstrated continuous airborne deployments for over a month at a time without returning to the ground.The concept of using LTA technology for energy generation is almost as old as LTA technology itself, and can be separated into three phases. The fi rst phase was Chris Vermillion ( ) · Ben Glass · Adam Rein Altaeros Energies