  Recent observations demonstrate that dwarf elliptical (dE) galaxies in
clusters, despite their faintness, are likely a critical galaxy type for
understanding the processes behind galaxy formation. Dwarf ellipticals are the
most common galaxy type, and are particularly abundant in rich galaxy clusters.
The dwarf to giant ratio is in fact highest in rich clusters of galaxies,
suggesting that cluster dEs do not form in groups that later merge to form
clusters. Dwarf ellipticals are potentially the only galaxy type whose
formation is sensitive to global, rather than local, environment. The dominant
idea for explaining the formation of these systems, through Cold Dark Matter
models, is that dEs form early and within their present environments. Recent
results suggest that some dwarfs appear in clusters after the bulk of massive
galaxies form, a scenario not predicted in standard hierarchical structure
formation models. Many dEs have younger and more metal rich stellar populations
than dwarfs in lower density environments, suggesting processes induced by rich
clusters play an important role in dE formation. Several general galaxy cluster
observations, including steep luminosity functions, and the origin of
intracluster light, are natural outcomes of this delayed formation.
