Three areas are especially important for his work: History of ethics, media ethics, and reconciliation studies. Since 2013, he is the Director of the Jena Center for Reconciliation Studies (JCRS), which is associated with the Faculty of Theology. The Center focuses on the theoretical foundation of trans-disciplinary and comparative studies on reconciliation, including the practical application of reconciliation models in conflict areas. Since 2009 he is the head of the International Summer School Series “Societies in Transition - Between Conflict and Reconciliation.” 

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In his own research, Leiner developed the “Hölderlin perspective” ,  This perspective assumes that reconciliation begins not only after the end of the conflict but already during and in the conflict, although it is challenged by violence and strife or enmity. Hence, conflicts are part of real life, and the task is to transform the conflict and use it as a challenge to create a more peaceful future. In this context, Leiner is the editor of the series “Research in Peace and Reconciliation” (RIPAR) at Vandenhoeck & Ruprecht (Göttingen), which offers case studies on conflict and reconciliation from various regions and continents. Currently, 7 volumes are planned, two of which were published in 2012 and 2014.​