This thesis explores how an action research approach based on a critical systemic perspective can benefit the practice of mediators in dealing with issues of justice during mediation processes. First, methodological reflections on critical systems thinking are presented, and a new development is proposed based on the ethics of Levinas. Also, a new synergy of methods and tools is developed. This brings together boundary critique, action science, statistics, system dynamics, alternative dispute resolution games, and interviewing. A description is then provided of how the methodology was used at a Colombian mediation centre. Here, the staff members and the author began the transformation of their professional mediation practice by reflecting on alternative perspectives on how they currently deal, and might deal in the future, with issues of justice. A critique was developed of several basic assumptions that are deeply ingrained in the mediation literature of the English speaking countries of the western world - in particular, that disputants are primarily concerned with their own private interests, and that mediation should therefore be considered successful if these interests are satisfied. In the mediation centre studied, most disputants prioritised justice principles over personal gain. Additionally, a new way of organising the interpretations of mediation presented in the literature is developed that can help mediation practitioners to be more conscious of the assumptions informing their professional practice. Finally, drawing upon both a literature review and the action research results, reflections are provided on the relevance of the notion of justice to mediation practice.