This document summarizes key issues regarding the development of modalities for REDD+ reference levels. It discusses the importance of reference levels for determining business-as-usual baselines and eligibility for results-based finance. Some technical issues addressed include the scope of REDD+, use of historical emissions data, adjusting for national circumstances, and linking reference levels to financing. Procedural issues discussed involve the adoption of reference levels under the UNFCCC and related processes under the Kyoto Protocol. The document provides principles and potential outlines for REDD+ reference level modalities to inform UNFCCC negotiations.