  We review the concept of the Fundamental Plane (FP), and present new results
on the near-infrared FP. We show that the K-band FP differs both from the
optical form and the virial expectation under the assumption of constant (M/L)
and homology. Systematic variations of only the stellar populations parameters
(age, metallicity, IMF) cannot reproduce the slopes of the FP simultaneously at
both the optical and near-infrared wavelengths. There appears to be an
additional effect which could be due to a systematic departure of the velocity
distributions of elliptical galaxies from a homologous family. In order to
distinguish between the different possible stellar populations parameters, the
FP and its projections can be observed at a range of redshifts. We describe
several such evolutionary tests: in the intercept of the color-magnitude
relation; in the intercept of the Kormendy surface brightness-radius
correlation; and in the intercept of the FP itself. All three tests are fully
consistent with the population of cluster elliptical galaxies having formed at
high redshift; this contradicts large, systematic age variations as a
significant contributor to the slope of the FP.
