Aluminosilicates are industrially indispensable materials with a far-reaching global commercial importance. Besides their more mundane use in construction materials and ceramics, their porous forms, most notably zeolites, represent the backbone of industrial heterogeneous acid catalysis. Zeolites are crystalline microporous aluminosilicate networks where some of the [SiO 4 ] tetrahedra are replaced by [AlO 4 ] -, necessitating charge compensation by a loosely bound cation in the pore space. Protonation gives rise to well-understood intra-framework Brønsted acid sites of the Si(μ 2 -OH)Al type, which were in the past believed to be responsible for all the catalytic properties of these materials. More recently, the presence and importance of Lewis acid sites have been uncovered and intensively studied. Such sites are not ordinarily present in freshly synthesized zeolites, but instead they are formed as extra-framework or frameworkassociated defects under the harsh conditions of high-temperature steaming and calcination -procedures used to increase the catalytic activities and hydrothermal stabilities of zeolites prior to industrial use. Evidence for a synergistic interaction between the Brønsted and the Lewis acid sites has also been presented. Direct characterization of the Lewis acid sites is challenging due to their extremely low volumetric abundance (near-surface species) and the high spin of the Al nucleus (I = 5/2), giving rise to a considerable quadrupolar moment which, in turn, interacts with the electric field gradient (EFG) to produce potentially very broad and complex line shapes. Although simple one-dimensional (1D) Al MAS NMR techniques are routinely used to observe highly symmetric species (e.g., [AlO 4 ] or [AlO 6 ]), which exhibit relatively narrow line shapes, other species of lower symmetry can result in resonances spanning several MHz (even tens of MHz), making them practically "invisible". The development of multiple-quantum (MQ) NMR techniques has recently allowed for direct observation and resolution of some of the low-symmetry species in zeolites. While direct observation of the hypothesized planar [Al(OSi) 3 ] remains elusive (predicted C Q = 34.8 MHz) and requires application of strong magnetic fields and ultrawide-line NMR experimental approaches , its presence in activated zeolites has been inferred through chemisorption of probe ligands (L), most notably pyridine (py), yielding resonances assigned as the corresponding [L-AlO3] species. The small quantity of the species of interest (compared to the total Al content of the materials) hinders more direct investigation of their properties.