Buckling in thin structures is generally considered as a first step towards failure. Instead, we view mechanical and interfacial instabilities in structures as opportunities for scalable, reversible, and robust mechanisms that must first be understood predictively, and then harvested for their function. This new design paradigm – building with instabilities – calls for an improved understanding of instabilities and pattern formation in complex media. Three examples will be presented: (1) fluid-instability based approaches for digitally fabricating geometrically complex uniformly sized structures, (2) flexible fabric-based gripper that contracts radially upon inflation (3) deployable structures inspired by insect wing expansion. The main feature common to these different problems is the prominence of geometry, and its interplay with mechanics, in dictating complex mechanical behavior that is relevant and applicable over a wide range of length scales.