In the last decade, World Trade Organization (WTO) Members have paid little attention to the WTO rulemaking processes and their functioning. Two high-levels commissions, as well as some scholars, have identified several areas of concerns with respect to the WTO rulemaking processes. Some of them have put forth proposals for their reform. However, the WTO has not proceeded with or even reflected upon any major reforms affecting the functioning of its rulemaking processes. The lack of attention by the Members regarding these issues motivated the focus of this thesis on the legitimacy of the WTO rulemaking processes. The principal research question of this thesis is: Are the WTO rulemaking processes legitimate? To what degree? Answering this first research question necessarily leads to a secondary one: How can the WTO rulemaking processes be assessed? This thesis recognizes that there is no uniform way for assessing legitimacy both at the national and international levels. It borrows from David Beetham's legitimacy conception and assesses the legitimacy of the WTO rulemaking processes from the standpoint of WTO Members. It builds a theoretical framework for assessing the legitimacy of the rulemaking processes on the basis of Members' conception of the WTO and the concepts of input and output legitimacy that have been frequently applied to the WTO and from which are derived four legitimacy criteria: legality, effectiveness, representativeness and openness. This thesis furthermore advances that legitimacy can only be effectively assessed as a matter of degree and, therefore, develops a multidimensional interval scale to allow a precise measurement of the four criteria of legitimacy as applied to the WTO rulemaking processes. In order to assess the rulemaking processes, it uses three cases that have led to the adoption of new rules or agreements. In fact, legitimacy matters even more for the processes that led to actual rules due to the fact that they generate binding outcomes. Such a methodology based on case studies arguably provides a more accurate representation of the WTO rulemaking processes than the general processes that have been described in the secondary literature.