The recent development of sensitive near-infrared detectors has opened up a new window for the study of high-redshift galaxies. Using the new near-infrared capabilities of the MOSFIRE and LRIS instruments at the Keck observatory, we have undertaken a spectroscopic survey of about 100 quiescent galaxies in the redshift range 1 < z < 2.5. Our deep spectra reveal the rest-frame optical absorption lines, and allow us to measure velocity dispersions and stellar ages. I will discuss our main results, including the comparison of dynamical and stellar masses, the link between local galaxies and their progenitors, and a solution for the puzzling size growth of quiescent galaxies.