Because of these weaker interactions, framework materials present large numbers of intramolecular degrees of freedom. Some of these degrees of freedom are relatively local and correspond, for example, to the large-scale motion or dynamics of a building block (in part or whole). Other degrees of freedom are cooperative and have large characteristic length scale, leading to dynamics at the scale of the entire unit cell, or largertherefore impacting the whole crystal, or at least nano-scaled domains inside a crystal. Although all molecular assemblies and solids show some degree of flexibility, evidence has accumulated that there is a propensity among framework materials to display large-scale dynamic behavior. This has previously been linked to their shared tendency to exhibit disorder, as well as the presence of defects in significant amounts. The first example in the literature of this striking behavior was probably the existence of flexibility triggered by adsorption, where materials like MIL-53 and MIL-88 could see their cell volume change by up to 40% upon adsorption of gas or 95% upon solvent uptake/removal, respectively. Later, materials were found that even surpassed these numbers, e.g., the expansion of unit cell volume in DUT-8(Ni) from the closed-pore phase to the open-pore phase reaching 254%. Since those seminal discoveries, the field has progressed rapidly and the number of reported cases of flexible framework materials has grown -as well as the extent to which the microscopic mechanism of their behavior has been studied, by a combination of experimental characterization techniques, in situ and in operando measurements, and computational simulation methods. We now understand much better why (and how) framework materials are frequently found to exhibit stimuli-responsive behavior, whereby the structure of a material is significantly changed upon chemical or physical stimulation and the modified state of the material induces a change in macroscopic properties. As depicted in Figure , this structural modification induces a change in the function of the material, or sometimes creates a new function. Because it introduces a coupling between different physical or chemical variables, these materials have been called by various names: multifunctional materials, smart materials, dynamic materials, soft crystals, etc. In this article, we will focus on what we believe is the simplest term: stimuli-responsive materials.