The modification of metal-organic frameworks (MOFs) in a postsynthetic scheme is discussed in this critical review. In this approach, the MOF is assembled and then modified with chemical reagents with preservation of the lattice structure. Recent findings show amide couplings, isocyanate condensations, 'click' chemistry, and other reactions are suitable for postsynthetic modification (PSM). In addition, a number of MOFs, from IRMOF-3 to ZIF-90, are amenable to PSM. The generality of PSM, in both scope of chemical reactions and range of suitable MOFs, clearly indicates that the approach is broadly applicable. Indeed, the rapid increase in reports on PSM demonstrates this methodology will play an increasingly important role in the development of MOFs for the foreseeable future (117 references).