Thermal bridges are highly conductive heat flow paths within building envelopes. The effects of thermal bridges on the overall heat loss through a building envelope are described. Thermally deficient areas caused by thermal bridges were found and their sizes ascertained in four office buildings by means of infrared thermography. Quantification of the heat loss caused by thermal bridging was achieved using field data obtained with heat flux transducers, along with a detailed analysis of the exterior thermograms and architectural drawings of the buildings involved. Field data were compared with the predictions obtained using a two-dimensional heat transfer model of the transient heat conduction within the exterior wall-floor system.