Major progress has been achieved in recent years in mapping the properties of
passively-evolving, early-type galaxies (ETG) from the local universe all the
way to redshift ~2. Here, age and metallicity estimates for local cluster and
field ETGs are reviewed as based on color-magnitude, color-sigma, and
fundamental plane relations, as well as on spectral-line indices diagnostics.
The results of applying the same tools at high redshifts are then discussed,
and their consistency with the low-redshift results is assessed. Most low- as
well as high-redshift (z~1) observations consistently indicate 1) a formation
redshift z>~3 for the bulk of stars in cluster ETGs, with their counterparts in
low-density environments being on average ~1-2 Gyr younger, i.e., formed at
z>~1.5-2, 2) the duration of the major star formation phase anticorrelates with
galaxy mass, and the oldest stellar populations are found in the most massive
galaxies. With increasing redshift there is evidence for a decrease in the
number density of ETGs, especially of the less massive ones, whereas existing
data appear to suggest that most of the most-massive ETGs were already fully
assembled at z~1. Beyond this redshift, the space density of ETGs starts
dropping significantly, and as ETGs disappear, a population of massive,
strongly clustered, starburst galaxies progressively becomes more and more
prominent, which makes them the likely progenitors to ETGs.Comment: To appear on Annual Review of Astronomy & Astrophysics, Vol. 44
(2006). 46 pages with 16 figures. Replaced version includes updated
references, few typos less, and replaces Fig. 11 and Fig. 16 which had been
skrewed u