Book excerpt: Metal−organic frameworks (MOFs) are a class of novel porous materials with a wide range of chemical applications. Structural characterization is important because understanding the relationship between the properties of these industrially relevant materials and their structures allows one to develop new applications and improve current performance. In this thesis 17O solid state NMR (SSNMR) spectroscopy is used to differentiate chemically and, under favorite circumstances, crystallographically nonequivalent oxygens and to follow the phase transitions in MOF-based materials. Several cost-effective approaches were used to prepare 17O-labelled MOFs. The metal centers of MOFs, including 47/49Ti, 67Zn, 91Zr, 115In and 139La, are also studied by SSNMR spectroscopy. It is demonstrated that SSNMR of metal ions is sensitive to the environment of metal cations. As a result, SSNMR is a useful tool to refine MOF structure, differentiate non-equivalent metal sites and probe the local structure of metal cations. SSNMR spectroscopy of the metal ions should be added to the arsenal of the techniques for MOFs characterization.