Metal-Organic Framework Composites - Volume I Materials Research Forum LLC Materials Research Foundations 53 (2019) 177-214 https://doi.org/10.21741/9781644900291-9 177 Chapter 9 Flexibility in Metal-Organic Frameworks: A Fundamental Understanding Christia Jabbour 2 † , Noor Aljammal 2, † , Tatjána Juzsakova 4 , Francis Verpoort 1,2,3,* 1 College of Arts and Sciences, Khalifa University of Science and Technology, PO Box 127788, Abu Dhabi, UAE. 2 Center for Environmental and Energy Research (CEER), Ghent University Global Campus, 119 Songdomunhwa-Ro, Yeonsu-Gu, Incheon, 406-840 South Korea 3 Laboratory of Organometallics, Catalysis and Ordered Materials, State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, China. 4 Institute of Environmental Engineering, University of Pannonia, Veszprem, 10 Egyetem St., 8200 Veszprém, Hungary. † Christia Jabbour and Noor Aljammal both authors have contributed equally to this work. *francis.verpoort@ghent.ac.kr Abstract Metal-organic frameworks (MOFs) had until recently the reputation of being one of the most critical porous materials. Their flexibility, however, has gained a lot of attention due to the wide selection of possible combinations between metal nods and/or ligands. Nonetheless, it is not always easy to identify the source of flexibility. This chapter focuses on the origin of flexibility, and the substantial geometrical changes that can occur due to external stimuli, such as temperature, pressure, light, gas or solvent adsorption. Flexibility control methods have also been discussed along with possible characterization techniques to help to identify the source of flexibility. Practical applications of flexible MOFs in gas separation and other processes are also discussed. In this respect, several prized examples covered by the literature are present to help in a comprehensive understanding in terms of design and structure tunability of flexible MOFs. Keywords Metal-Organic Frameworks, Flexibility, Mechanical Properties, Secondary Building Unit, Characterization