Janus membranes are an emerging class of materials having opposing properties at an interface. This structure results in selective and often novel transport characteristics. In this Minireview, a definition of the Janus membrane, beyond merely asymmetric materials, is introduced and common fabrication strategies are outlined. Also presented are current and potential applications in directional transport, switchable permeation, and performance optimization with detailed mechanisms. Crossing over: Janus membranes have received widespread interest over the past years. In this Minireview, a special definition of a Janus membrane is proposed and common fabrication methods of such membranes are outlined. Also summarized are the current and potential applications of Janus membranes in directional transport, switchable permeation, and performance optimization.