Residual stresses which arise from thermal expansion and contraction due to welding may have contributed to the brittle fracture exhibited by welded steel beam-to-column connections during the Northridge Earthquake. These residual stresses have a strong influence on crack initiation and crack propagation in the vicinity of stress concentrations (i.e., unfused backup bar in welded steel beam-to-column connections) and account for changes in the driving force for fracture. They affect material toughness by changing the constraint condition under which fracture occurs. Currently, all methods of dealing with residual stresses are hampered by the lack of a consistent means of measuring the magnitudes and distribution of these stresses. This paper describes a new acoustic microscopy technique that allows the mapping of residual stresses in welded connections with high spatial resolution. The technique is based on the sensitivity of polarized acoustic modes to local elastic anisotropy induced by stress. The technique furthermore allows the mapping of residual stresses in a "tomographic" way by changing the frequencies of the acoustic waves. The results reveal that the magnitude of the residual stresses is influenced by the local microstructure of the steel and the weld metal. Ductile microstructures within the weld and the heat affected zone release residual stresses by yielding, whereas brittle microstructures retain residual stresses.