Three open problems in our understanding of early-type galaxies are 1) identifying the
process(es) responsible for their rapid size evolution, 2) accurately constraining the stellar
IMF and its variations in the population, 3) measuring the density profile of their dark
matter halo. We use strong lensing as the main diagnostic tool to address these issues.
We first dissected a massive elliptical galaxy in its stellar and dark matter components,
measuring both its IMF and the inner slope of the dark matter halo. We then collected
a sample of 45 strong lenses in the redshift interval 0.2 < z < 0.8 and used them, in
combination with lenses from other surveys, to measure the slope of the total density
profile, the stellar IMF and the dark matter mass in the population of massive early-type
galaxies, and their time evolution. Finally, we used our measurements of the evolution of
the density slope to test a galaxy growth scenario based on purely dissipationless mergers.
Our main results are: the stellar IMF of massive early-type galaxies is significantly heavier
than that of the Milky Way and correlates with galaxy mass; the dark matter halo has
a steep slope in at least one system; more compact galaxies have less dark matter than
their extended counterparts at fixed redshift and stellar mass; early-type galaxies evolve
while keeping the slope of their total density profile approximately constant. This last result cannot be reproduced with purely dissipationless mergers, therefore a little amount
of dissipation is required.