Observations of galaxy pairs, disturbed morphologies, and close interactions indicate that the major galaxy merger rate rises from z = 0 to around z ~ 1–2 but then shows, at most, a weak further increase or even a plateau and decline beyond z ? 2, contrary to some ΛCDM-based expectations of a steeply rising merger rate at earlier times driven by rapidly growing dark matter halo merger rates (Conselice 2014; Dalmasso et al. 2024). Semi-analytic and hydrodynamic ΛCDM models can be tuned to match parts of the data, but the combination of relatively modest high-z merger fractions, the abundance of massive disks, and the detailed redshift evolution of merger indicators suggests tensions between simple hierarchical merging prescriptions and the observed assembly history of galaxies (Lotz et al. 2011; Conselice 2014).