Power ultrasonics can precisely introduce vibrational energy into materials and the interfaces of two or more joining partners within fractions of a second. Ultrasonic welding is based on this ability to commonly join single and multi-materials such as polymers to other polymers in the packaging industry or metals to the same or other metals for conductive joints in electronics. Research investigates many facets of ultrasonic welding including the oscillation units and tools as well as the possible material combinations and their properties such as metal and carbon fibre reinforced (CF) thermoplastics. In addition, power ultrasonics are sometimes added to other processes to improve the effectiveness of another process, e.g. reduction of friction during machining or extraction. This contribution presents the manufacture of single and multi-material joints by power ultrasonics, focussing on the interface formation during joining and its correlation to the mechanical strength and the fracture behaviour of the joints. Several selected joints will be presented including aluminium/copper, titanium/aluminium and titanium/CF-PEEK. Another process has recently been developed to form and join three-dimensional single material bodies made from carbon fibre reinforced thermally reversible polymers. The performance in terms of joint strength highly depends on the used tools and the process parameters that induce just the right amount of energy into the joining zone to form favourable interfaces without damaging the individual materials. Details of the processes are presented as well as the examination of the samples by optical microscopy, scanning electron microscopy and computer tomography. Complementary, finite element simulations of loaded joint interfaces will be introduced, which have been developed to understand the deformation and fracture behaviour of joints and to predict favourable interface geometries.