In recent years, several surveys have shown that massive galaxies has
undergone a major evolution during the epoch corresponding to the
redshift range 1.5-3, assembling a significant fraction of their
stellar mass in this epoch.  To understand the origin of this rapid
rise, a closer scrutiny on the nature and physical properties of
massive galaxies at high redshift is needed.  I will present our
recent results based on the analysis of the 24micron MIPS data of the
GOODS-S field, that allow to trace star-formation (or the lack of) in
high redshift galaxies without biases due to dust extinction.  
I will show the results of our analysis focusing in particular on:

- the accuracy of the estimate of the SFR from 24micron data, and the
comparison with other estimators;
- the evolution of the Specific star formation rate as a function of
redshift and stellar mass;
- the fraction of quiescent and "red&dead" galaxies as a function of
redshift.

The scenario emerging from these data will be compared with recent
predictions of theoretical models, to discuss the validity of their
physical ingredients.