Quasars are the most luminous active galaxies powered by an accreting supermassive BH. The fact that massive BHs not only exist in the centers of active galaxies but also in inactive normal galaxies shows that more than the mere presence of a massive BH is needed to trigger the AGN activity. One promising candidate is the sudden inflow of gas to the center induced by mergers. Numerical simulations have shown that remnants of galaxy mergers often closely resemble elliptical galaxies and indeed, the majority of quasars hosts look like ellipticals. However, the connection between mergers and quasar activity is still strongly debated: Did the host galaxies form at high redshift and are now dominated by old bulges or are there signs of recent mergers as in the case of ultra-luminous infrared galaxies? To address this question, we obtained very deep HST/ACS images of five quasars. All objects reveal strong signs of interactions (shells, tidal tails, warped disks), one spectacular example being MC2-1635+119. We compare our results with numerical simulations of galaxy mergers to estimate the type and age of the merger. Our results show that at least some quasar host galaxies are the products of relatively recent merger events rather than old galaxies which formed at high redshift.