Although a current hot topic, tangible user interfaces (TUIs) have been studied for quite some time. One particular kind of TUI has been investigated repeatedly: during the past 20 years several groups have developed ıntelligentbuilding blocks that can self-describe the geometric structures into which they are assembled. These computational construction kits (CCKs) give users an extremely intuitive way to express 3D geometry, and have been used for a variety of different geometric modeling and design applications. In this panel we will review the development of CCKs, from the first such systems to the most recent, and speculate about future developments. The panelists will present and demonstrate artifacts from several CCKs, most of which have not been shown previously at SIGGRAPH, CHI, UIST, or I3D.