This paper discusses a number of special ultrasonic NDE problems of particular interest to inertia and friction welds, but most of the results can be readily applied to other similar or dissimilar solid-state bonds as well. One of the major problems associated with dissimilar bonds is the “blinding” effect of the strong interface reflection caused by the inherent elastic discontinuity at the otherwise flawless boundary. A novel signal-processing technique based on the symmetric part of the boundary reflections from the two opposite sides of the interface is introduced and shown to yield a quantitative measure of boundary imperfection even for very good, apparently flawless bonds. The second problem addressed in this paper is the so-called “cold-weld” effect. As a combined result of heat softening and external pressure, intimate mechanical contact can be achieved without real fusion. Frequency analysis of the reflected ultrasonic signal from such cold-welded areas is shown to be capable of measuring the degree of plastic deformation at the contacting surfaces and, indirectly, to assess interface pressure and temperature. The third problem considered in this paper is the ultrasonic evaluation of the characteristic layered distribution of the grain structure in flawless inertia and friction welds. As an example, the thickness of the heat-affected zone (HAZ) was determined by ultrasonic spectroscopy, and the results were found to be in good agreement with independent optical profile measurements. Furthermore, the HAZ thickness is shown to be strongly correlated to welding pressure, therefore it offers a sensitive indicator for bond quality assessment as well.