**ABSTRACT****e-Main****-Seque**

Early--type
galaxies (ETGs) are known to populate a two-dimensional manifold in
the space of their observed quantities. This

Fundamental Plane (FP) is usually expressed by a correlation among the
logarithm of the effective radius log r_e, the mean surface brightness
<mu>_e within r_e, and the logarithm of the central velocity dispersion
log sigma_0. I will discuss the main properties of the FP, with regard,
in particular, to the so-called TILT of the relation with respect to
the predictions of the Virial Theorem for an homologous family of galaxies
with constant mass to light ratio. The most recent observational/theoretical
results will be described.