Various terms have been used to describe framework materials that can undergo large-scale changes in their structure, such as dynamic or flexible materials, stimuli-responsive frameworks, or soft porous crystals. The number of such materials has been growing, and include many members from supramolecular families such as the now ubiquitous metal-organic frameworks (MOFs), 18 but also covalent organic frameworks (COFs) and supramolecular organic frameworks (SOFs). While the common occurrence of this flexibility induced by guest adsorption or solvent removal was noted relatively early in the development of MOFs and related materials, it was only later that researchers in the field fully realized that soft porous crystals often also exhibit a structural response to other physical or chemical stimuli.