  We use semi-analytic techniques to study the formation and evolution of
brightest cluster galaxies (BCGs). We show the extreme hierarchical nature of
these objects and discuss the limits of simple ways to capture their evolution.
In a model where cooling flows are suppressed at late times by AGN activity,
the stars of BCGs are formed very early (50 per cent at z~5, 80 per cent at
z~3) and in many small galaxies. The high star formation rates in these high-z
progenitors are fuelled by rapid cooling, not by merger-triggered starbursts.
We find that model BCGs assemble surprisingly late: half their final mass is
typically locked-up in a single galaxy after z~0.5. Because most of the
galaxies accreted onto BCGs have little gas content and red colours, late
mergers do not change the apparent age of BCGs. It is this accumulation of a
large number of old stellar populations -- driven mainly by the merging history
of the dark matter halo itself -- that yields the observed homogeneity of BCG
properties. In the second part of the paper, we discuss the evolution of BCGs
to high redshifts, from both observational and theoretical viewpoints. We show
that our model BCGs are in qualitative agreement with high-z observations. We
discuss the hierarchical link between high-z BCGs and their local
counter-parts. We show that high-z BCGs belong to the same population as the
massive end of local BCG progenitors, although they are not in general the same
galaxies. Similarly, high-z BCGs end-up as massive galaxies in the local
Universe, although only a fraction of them are actually BCGs of massive
clusters.
