Metal-Organic Frameworks (MOFs) are a class of coordination polymer in which metal ions and organic electron donors (“linkers”) assemble to form crystalline structures with stable nanoporosity. As a result of well-understood principles governing inorganic coordination chemistry and the rigidity and topology of the linkers, rational design principles have been established that allow MOFs to be designed and synthesized with properties selected for specific applications. Among the thousands of known MOFs, those with incomplete coordination shells around the metal ion (also known as open metal sites, or OMS) exhibit unique properties, including increased uptake of weakly interacting gases such as H₂ and CH₄, catalytic behavior resulting from substrate coordination to the OMS, and guest-dependent electrical conductivity that provides an avenue to electronic devices and sensors. This presentation will provide an introduction to MOFs and MOFs with OMS, then illustrate their possibilities using examples from our recent research: 1) catalytic cleavage of carbon-oxygen bonds found in lignin using activated H₂; 2) colorimetric sensing; and 3) creation of electrically conducting MOFs using redox-active guest molecules such as TCNQ (7,7,8,8-tetracyanoquinodimethane). In each of these examples, guest molecules in the MOF pores interact directly with the OMS, resulting in bond polarization, modification of the electronic structure of the metal ion, and/or charge transfer between the metal ion and the guest. In the case of C-O bond cleavage, chemisorption of substrate ether molecules to Mg(II) ions in the IRMOF-74 series weakens the C-O bond. In addition, the MOF activates H₂ molecules in solution with the substrate, as shown by isotope exchange experiments. Colorimetric sensing is enabled by guest binding to Cu(II) ions in the MOF known as HKUST-1, resulting in new charge-transfer transitions in the visible and near-UV. These perturbations of the electronic structure are relatively minor compared with the binding of TCNQ, a pi acid that bridges between two dimeric copper units in HKUST-1, creating a pathway for charge transport via a hopping mechanism. These results demonstrate that the novel features of MOFs with OMS provide the means to achieve completely new classes of catalysts and materials for chemical sensing and other electronic devices.