Extensive work has been devoted recently to the development of 3D printing or additive layer manufacturing technologies, as well as to the field of flying robots. However, to the best of the authors7 knowledge, no robotic prototype has been presented so far that combines additive layer manufacturing techniques with aerial robotics. In this paper, we examine the feasibility of such a hybrid approach and present the design and characterisation of an aerial 3D printer; a flying robot capable of depositing polyurethane expanding foam in mid-flight. We evaluate various printing materials and describe the design and integration of a lightweight printing module onto a quadcopter, as well as discuss the limitations and opportunities for aerial construction with flying robots using the developed technologies. Potential applications include ad-hoc construction of first response structures in search and rescue scenarios, printing structures to bridge gaps in discontinuous terrain, and repairing damaged surfaces in areas that are inaccessible by ground-based robots.