Conventional dense materials are expected to expand in response to heating, however, a number of materials display the opposite behavior, shrinking with increasing temperature. This phenomenon, known as negative thermal expansion (NTE), is considered counterintuitive and is observed in only few inorganic solids and certain classes of framework materials. The technological importance of NTE relies on the application of this thermal volume contraction to negate the positive thermal expansion of conventional materials. For example, this control of thermal expansion is crucial in materials used for dental fillings. A number of physical features can cause NTE. These include phase transitions, transverse vibrational modes and rigid unit modes. For example, ZrW 2 O 8 shows a remarkable contraction between 0.3-1050 K owing to coupled rotations of rigid unit vibrational modes. NTE appears to occur more frequently in framework materials. In particular, several families of zeolites exhibit NTE. Metal-organic frameworks (MOFs) are porous materials, subject to increasing attention, that can produce a wide variety of porous framework structures through the combination of organic ligand units and metal nodes. Many MOFs exhibit NTE, including the prototypical MOF-5 and UiO-66. MOFs have advantages over traditional dense solids due to their high porosity, with large voids encapsulated by the structure. Mesoporous MOFs, with pore diameters larger than 20 Å, have structures that appear to be well suited for NTE. Firstly, owing to their large free volume, enabling massive contractions. Secondly, the large molecular subunits are expected to produce transverse phonon modes with increasing temperature, resulting in contraction.