  In their study of the evolution of galaxies within clusters, Butcher and
Oemler discovered evidence for a strong evolution in star-formation rate with
redshift. Later studies confirmed this conclusion and uncovered several aspects
of the effect: photometric, spectroscopic, and morphological. This article
reviews a broad sample of these works and discusses selection effects, biases,
and driving mechanisms that might be responsible for the changes in
star-formation rate.
