In parliamentary systems opposition groups are seen as residual, having very little influence on the policy making process. However, some parliamentary groups might be more effective than others at inserting their proposals into enacted legislation. Due to data and methodological limitations, large-N and systematic studies of the conditions under which parliamentary groups are successful at influencing legislation are extremely rare, leaving crucial questions about who and why has power unanswered. Do parliaments reverse executive proposals? What groups are more efficient in performing this task? In order to answer these questions, first, we compare executive bills proposals as they enter the legislative process with the final text of laws using text reuse methods. Next, we identify the parliamentary group responsible for the different amendments and calculate the amendments’ efficiency rate for each group. The theoretical framework takes into consideration the macro level (the characteristics of bills, type of government, electoral cycle), but also the micro level (parliamentary groups’ resources, past parliamentary experience, specialization and ideological distance from the incumbent). The analysis relies on a database containing all the executive bills introduced into the Spanish Parliament from 1986 to present.