Observations have identified several extremely large groupings of quasars (quasi-stellar objects, which are powered by supermassive black holes in distant galaxies) spanning spatial scales of 300-600 million light-years or more. These large quasar groups are problematic for Lambda-CDM because quasars are rare objects, and the probability of randomly finding so many quasars within such enormous projected structures is extremely low according to the standard model. The existence of these coherent, large-scale quasar associations suggests either that quasars cluster far more strongly than expected, or that they trace underlying large-scale structure in ways not predicted by Lambda-CDM structure formation. The implied correlation lengths and clustering patterns exceed what standard models predict for objects at such high redshifts, challenging assumptions about the homogeneity of the universe and the clustering properties of rare objects (Clowes et al. 2012; Lietzen et al. 2011).