This paper addresses recent progress in studies of collisional depolarization of rotational angular momentum in open-shell molecules. The application of the polarization spectroscopy (PS) technique to the OH radical in collisions with the rare gases He and Ar is described. The capacity of quantum mechanical scattering calculations on ab initio potential energy surfaces to reproduce the experimental results is considered. We explain how the very recent progress in theoretical methodology has enhanced the value of this comparison, reflect more generally on the relationship to other experimental methods and molecular systems, and speculate on some promising future directions for this research field. © 2009 The Royal Swedish Academy of Sciences.