3D IC solutions have been developed for several different reasons: to reduce the system form factor for portable platforms; to increase system performance by alleviating the interconnect-delay bottleneck; and to manage overall system cost by stacking heterogeneous chips, rather than integrate diverse system components into a single chip through 2D scaling. However, although some 3D IC markets are emerging, and most technical issues of 3D integration are almost solved, several thermal and production-test challenges remain as obstacles. This special issue of IEEE Design & Test takes a look a those issues.