Wind tunnels continue to be used extensively for the development of aerospace vehicles and ground vehicles.  An area seeing continued growth is in aero-acoustic testing.  For aeronautical vehicles, primarily fixed wing aircraft and rotorcraft, the emphasis is on external noise sources.  For ground vehicles, primarily cars and SUVs, the emphasis is on interior noise sources.  The main focus of this paper is a general description of the aero-acoustic aspects of wind tunnels.  Both open-jet and solid wall wind tunnels will be discussed.  First, the genesis of the in-flow and out-of-flow sound pressure level specifications, over the past 20 years, will be outlined.  Design concepts used to satisfy the acoustic specifications will be enumerated with examples.  Finally, resonance and other acoustic issues will be discussed briefly.  The solution techniques used to ameliorate the acoustical issues will be highlighted in this paper.