This thesis focuses on numerical analysis of damage development and ductile failure in welded joints. Two types of welds are investigated here. First, a study of the localization of plastic flow and failure in aluminum sheets, welded by the relatively new Friction Stir (FS) Welding method, has been conducted ([P1], [P2], [P7]-[P9]). The focus in the thesis is on FS-welded 2xxx and 6xxx series of aluminum alloys, which are attractive, for example, to the aerospace industry, since the 2024 aluminum in particular, is typically classified as un-weldable by conventional fusion welding techniques. Secondly, a study of the damage development in Resistance SpotWelded joints, when subject to the commonly used static shear-lab or cross-tension testing techniques, has been carried out ([P3]-[P6]). The focus in thesis is on the Advanced High Strength Steels, Dual-Phase 600, which is used in for example, the automotive industry due to its good mechanical properties. Both welding techniques are known to result in a significant change of the microstructure in the weld region. Thus, some experimental investigations have been conducted to estimate the variation of the model parameters across the weld as well as to obtain experimental measurements for comparison with the developed models ([P3], [P7]-[P9]). However, the main focus in this thesis is on modelling the large material deformation in the weld region that eventually leads to ductile failure, as loading is applied. All numerical models developed in this thesis are based on the classical micromechanical Gurson model (Gurson-Tvergaard-Needleman model), which approximates the ductile failure mechanism by nucleation, growth and coalescence of spherical micro-voids through a set of constitutive equations. Extensions to this classical model that account for the void shape evolution (the Gologanu-Leblond-Devaux model) and failure during low triaxiality shearing (the Nahshon-Hutchinson shear modification) have also been applied to predict failure in welded joints.