The study classifies and isolates the effects of thermal wake interference that occurs in multilouvered fins. Inter-fin interference occurs between adjacent rows of louvers and is dominant at high Row efficiencies when the how is louver directed. Intra-fin interference occurs on subsequent louvers of the same row or fin and is dominant at low flow efficiencies or when the Row is predominantly duct directed. It is established that thermal wake effects carl be expressed quantitatively as functions of the flow efficiency and the fin pitch to louver pitch ratio. While the heat transfer capacity of multilouvered furs increases unconditionally when thermal wakes are eliminated, the heat transfer coefficient either increases or decreases depending on tile relative location of thermal wakes in the vicinity of louvers. Experimental procedures, which neglect thermal wake effects in determining the heat transfer coefficient, while not introducing large errors at high flow efficiencies, call introduce errors as high as 100% at low flow efficiencies.