3D characterization is growing quickly in materials science due to the demands of better microstructural characterization, which cannot be fully achieved with the traditional 2D microscopy techniques. In this work, two types of 3D characterization techniques were employed: MicroCT – microcomputed x-ray tomography (with both bench top and synchrotron sources) and FIB-SEM (focused ion beam coupled to SEM). These techniques were applied to a specific system: discontinuities in underwater wet welds. These discontinuities (pores, cracks and inclusions) range in size from nanometers to tens of microns. Moreover, they present complex and varied shapes, spatial distribution and orientation. Thus, this thesis presents the development of methodology for the acquisition, processing, analysis and visualization of pores, cracks and inclusions in underwater wet welds, from images obtained by MicroCT and FIB-SEM. The acquisition techniques and conditions were optimized for the different kinds of discontinuities. Specialized routines for image processing and analysis were developed, employing a free software environment whenever possible (FIJI/ImageJ). Several measurements were automatically obtained: number of objects, volume, volume fraction, surface area, feret diameter, thickness, sphericity and compacity. Moreover, the rendering of 3D images allowed the observation of the shape and spatial distribution of the discontinuities in the weld metal. To evaluate the detection sensitivity of cracks by MicroCT, a specimen with varied cross-sections was submitted to a tensile test, so that the different sections were submitted to to different stress values. A positive correlation was observed between the stress value and the number, length and thickness of the detected cracks. This experiment also showed the influence of spatial resolution and noise upon the possibility of detecting cracks accurately.