The resulting segregation of large RE 3+ and small Al 3+ onto the A and (B, C) sublattices respectively is associated with a low tolerance to deviation from ideal (3:5) stoichiometry. For example, in Y3Al5O12 (YAG), the most energetically favourable way to deviate from the nominal stoichiometry is to substitute Y 3+ for B site Al 3+ , but such substitutions are difficult to realise in appreciable concentrations, with an estimated limit of x < 0.03 in conventionally-synthesised YAG ceramics. This behavior is common to all members of the RE3Al5O12 series and means that rare-earth dopants used to introduce and control luminescence properties are effectively constrained to a single crystallographic sublattice (A) with similar local coordination environments around each dopant ion. The luminescence properties of the phosphors are thus controlled by the identity of the dopant(s), with little scope for further property tuning via control of dopant distribution within the host structure. However, if large deviations from 3:5 stoichiometry can be imposed on the host structure, it becomes possible to distribute significant concentrations of dopant ions over multiple cation sublattices, representing an extra degree of freedom for property tuning. We recently demonstrated this effect in a series of highly nonstoichiometric YAGs (Y3+xAl5-xO12 with 0 ≤ x ≤ 0.40), which can be isolated by crystallisation of glass or deeply-undercooled melts. In these materials, excess Y 3+ is accommodated by substituting up to 20% of the Al 3+ at the B sublattice. This allows small RE 3+ dopants such as Yb and Er 3+ to be distributed over the A and B sublattices, with a strong effect on the color of their upconversion emission. The extension of this concept to non-YAG garnets (RE3+xAl5-xO12) could have significant advantages, but it also presents certain challenges. Notably, a wider range of applications can be addressed by changing RE 3+ . For example, as well as their luminescence properties (that are complementary to those of YAG), RE3Al5O12 garnet aluminates based on heavier rare-earths such as Lu3Al5O12 are particularly suitable for use as scintillators; 3 while the Gd3Al5-xGaxO12 family hosts excellent persistent luminescence properties; and the various open-shell 4f x configurations support different magnetic phenomena and applications. The challenges to this development relate mainly to synthesis and crystal chemistry: the extent of crystallographic site-sharing between Al 3+ and Y 3+ in Y3+xAl5-xO12 is already surprising due to their large size contrast, 7 but several of the garnet-forming rare earth ions have an even larger radius than yttrium. Conventional crystallo-chemical assumptions suggest that these should be even less amenable to non-stoichiometry. Like YAG, nonstoichiometric RE3+xAl5-xO12 are expected to be metastable, and should require an appropriate synthesis route that avoids extended annealing times at very high temperatures. It is clear that, before the potential of RE3+xAl5-xO12 functional materials can be realized, it is important to understand which RE 3+ systems are amenable to extreme nonstoichiometry, where their compositional and thermal stability limits lie, and how to synthesize them.