here is a long-lasting tradition in political science that draws a sharp dividing line between government and opposition parties on the ground that while governing parties have to focus on responsibility, the opposition parties– and above all, those permanently in opposition – are irresponsible per se as they are not made accountable for their decisions (Sartori 1966; Mair 1997). This assumed duality has already been questioned (Kriesi 2014), but has not been fully followed up in either theoretical or empirical respects. The irresponsibility of the opposition is both overstated and under-researched. Obviously, the opposition parties are also accountable at the election, but their responsibility cannot be based on how many pledges they fulfil, as they do not lead the decision-making process. Still, we claim that opposition parties’ responsibility can also be expected, measured, and analysed. The main goal of this paper will be a better (and empirically oriented) specification of what “responsibility” is when we look at it from different perspectives (i.e., from both the government and the opposition parties’ side). How do the government and opposition parties fulfil their responsibility in terms of expected duty through their parliamentary activity? The answer could be built up in two dimensions: one institutional and one policy related. At the institutional level, we will explore to what extent government and opposition parties use the available parliamentary opportunity structures. At the policy level, we will see whether opposition parties react to the government initiatives and/or whether they attempt to carry out an agenda of their own. We expect variation both between government and opposition parties and within the opposition camp, due to a series of variables linked to the party type but also to the characteristics of the executives. We will test these expectations in three different countries: Hungary, Italy and Spain.