The aim of this paper is to offer a number of elements for analysis and reflection on parliamentary opposition in a comparative perspective. More specifically, it identifies four sets of explanatory factors with which to analyze the relative strength of parliamentary opposition: the structure of the regime and the institutional position of the legislature in the political system, the legal and/or procedural devices granted to the opposition in the parliamentary arena, the informal practices of the opposition, and finally, the characteristics of the lawmakers. Taking these dimensions into account should help determine the relative strength of parliamentary opposition, its ability to act and its influence, and should lead to empirical measures of the variation found between different countries.