The European Commission, under Directorate-General for Environment (DG ENV) created an ad hoc group of Commission Services and Member States to serve the EU Community Strategy on Endocrine Disrupters. The ad hoc group created the Endocrine Disrupters Expert Advisory Group (ED EAG) in November 2011 to provide detailed reflections on scientific issues relevant to identifying and assessing endocrine disrupting substances, not specific to any regulatory framework. The European Commission's Joint Research Centre was tasked with facilitating and chairing meetings of the ED EAG and preparing this report. The scope of the present report is to capture the experts' opinions on scientific issues relevant to the likelihood of the existence of thresholds for a biological response of an organism to an ED, in particular considering thresholds of adversity and the uncertainties associated with reliably estimating such thresholds from experimental data. The question although relevant to the evaluation of EDs per se was specifically raised by the ad hoc group in relation to a review of the REACH regulation with respect to treatment of EDs under authorisation (Article 138 (7)).