  We consider the observational constraints on stupendously large black holes
(SLABs) in the mass range $M \gtrsim 10^{11}\,M_{\odot}$. These have attracted
little attention hitherto and we are aware of no published constraints on a
SLAB population in the range $(10^{12}$ - $10^{18})\,M_{\odot}$. However, there
is already evidence for black holes of up to nearly $10^{11}\,M_{\odot}$ in
galactic nuclei, so it is conceivable that SLABs exist and they may even have
been seeded by primordial black holes. We focus on limits associated with (i)
dynamical and lensing effects, (ii) the generation of background radiation
through the accretion of gas during the pregalactic epoch, and (iii) the
gamma-ray emission from the annihilation of the halo of weakly interacting
massive particles (WIMPs) expected to form around each SLAB if these provide
the dark matter. Finally, we comment on the constraints on the mass of
ultra-light bosons from future measurements of the mass and spin of SLABs.
