Abstract: A heat and humidity exchanger comprises a pleated membrane cartridge disposed in a housing. The cartridge comprises a pleated water-permeable membrane via which heat and humidity can be transferred between two fluid streams. A flow field element is disposed within some or all of the folds of the pleated membrane, for directing the stream over the inner surfaces of the folds. The flow field path defined by the flow field element enhances flow distribution across one or both membrane surfaces, controlling the relative flow paths of the two streams on opposite sides of the membrane and reducing the pressure drop across the heat and humidity exchanger. The flow field elements provide improved water transfer and allow for a more compact device. The flow field elements can also assist in supporting the pleated membrane and controlling the pleat spacing within the pleated membrane cartridge. The heat and humidity exchanger is particularly suitable for fuel cell and energy recovery ventilator (ERV) applications.