Negative gas adsorption (or NGA) falls into the category of adsorption-induced flexibility, which we have covered above, as well as counter-intuitive behavior. Reported for the first time in 2016 by the Kaskel group in Dresden, NGA materials feature a decrease in their adsorbed uptake upon increase of gas pressure during an adsorption isotherm, i.e., they have (δN ads /δP) T < 0. This feature, which is thermodynamically impossible in systems at equilibrium, is contrary to adsorption in "normal" microporous solids -where the guest uptake always goes up when the pressure in the surrounding reservoir is increased, displacing the equilibrium towards more molecules inside the pore space of the adsorbent. It is exemplified in the case of prototypical NGA material DUT-49 in Figure . NGA was shown to be due to the combination of two factors: (i) the existence of an adsorption-induced contraction of the material, from an open-pore phase to a narrow-pore phase; (ii) an important metastability of the open-pore structure, which means that the closing transition is delayed past the point of phase equilibrium. Beyond this thermodynamic understanding of the nature of the NGA transition, more recent works have focused on the engineering of negative adsorption in other materials. used an extensive combination of in situ X-ray and neutron diffraction, NMR spectroscopy, calorimetric and adsorption measurements, alongside molecular simulations to highlight a series of critical design criteria for NGA: limits on pore size, ligand length and mechanical stiffness, strength of guest adsorption, etc.