A major goal of contemporary astrophysics is understanding the origin of the
most massive galaxies in the universe, particularly nearby ellipticals and
spirals. Theoretical models of galaxy formation have existed for many decades,
although low and high redshift observations are only beginning to put
constraints on different ideas. We briefly describe these observations and how
they are revealing the methods by which galaxies form by contrasting and
comparing fiducial rapid collapse and hierarchical formation model predictions.
The available data show that cluster ellipticals must have rapidly formed at z
> 2, and that up to 50% of all massive galaxies at z ~ 2.5 are involved in
major mergers. While the former is consistent with the monolithic collapse
picture, we argue that hierarchal formation is the only model that can
reproduce all the available observations.Comment: Invited Review, 10 pages, to appear in "Galactic Dynamics", JENAM
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