Discusses the issues which influence the performance of heat recovery devices within typical building applications. It is intended to cover the three main types if devices installed in ventilation systems in residential and commercial buildings, which are run-around coils, plate heat exchangers and thermal wheels (rotary regenerators). Other systems such as heat pipes are also described briefly. The successful implementation of air to air heat recovery depends on factors such as careful equipment selection, proper integration of equipment into the ventilation system, good commissioning, proper maintenance and operation, and building characteristics, such as airtightness. One section gives information to assist the designer and building operator to understand some of the factors that influence the performance of heat recovery systems, highlighting some of the key points that have emerged from the analysis of existing information, detailed simulation work and site measurements.