  The K-band Hubble diagram of Brightest Cluster Galaxies (BCGs) is presented
for a large, X-ray selected cluster sample extending out to z = 0.8. The
controversy over the degree of BCG evolution is shown to be due to sample
selection, since the BCG luminosity depends upon the cluster environment.
Selecting only the most X-ray luminous clusters produces a BCG sample which
shows, under the assumption of an Einstein-de Sitter cosmology, significantly
less mass growth than that predicted by current semi-analytic galaxy formation
models, and significant evidence of any growth only if the dominant stellar
population of the BCGs formed relatively recently (z <= 2.6).
