Design of tangible interactive systems (TISs) involves understanding and taking into account stability issues. What do we mean by TIS stability? Any time a new artifact or system is designed, we need to question impact of its use in our society and environment. We need to question social stability and environmental stability with respect to technological evolution and consequent emergence of new practices. Stability issues naturally follow maturity issues that were presented and discussed in the previous chapter. They include important concepts such as observability, controllability, redundancy, cognitive stability, cognitive support, resilience, safety, and security of life-critical systems. Let's first remind old concepts and define new ones.