International tribunals’ receptivity to international environmental law varies. The thesis provides a comprehensive framework for empirically assessing the receptivity of various international tribunals to international environmental law. The underlying assumption is that receptivity promotes international environmental law evolution. Receptivity concerns the openness of tribunals to international environmental law considerations. This is the first time to introduce a definition of receptivity in the literature. Receptivity is defined as the tribunal’s openness, interest, sensitivity and responsiveness to consider or accept ideas, obligations and principles related to international environmental law. Several considerations impact receptivity so the thesis identifies three drivers – systemic, institutional and idiosyncratic – each comprised by different indicators, that can be objectively identified on the face of the rulings across tribunals over time. The thesis examines selected ruling from the ICJ, ITLOS, human rights courts, the WTO and investment tribunals. The analysis shows that indicators guide the drivers and that it is the interactions of the three drivers that determines final receptivity of the tribunal. This framework for the assessment of receptivity shows that there are degrees of receptivity and such degrees depend on the direction of the three drivers of receptivity. It allows for the ranking of the tribunals as more or less receptive to international environmental law and its reassessment over time.