This document is one of two Resource Papers (the other is on Limits to What Can Be Offset) written to update and complement information already published in the Offset Design Handbook (BBOP, 2009) and to support the interpretation and understanding of the Principles, Criteria and Indicators being developed for the BBOP Standard on Biodiversity Offsets (BBOP, 2012). The document specifically addresses Principle 4 (No Net Loss, ‘NNL’), although the interpretation of NNL is relevant to most of the ten BBOP principles. The paper outlines the key issues that need to be considered in working towards the goal of biodiversity offsets – a NNL or net gain outcome for biodiversity. It sets out a broad conceptual framework for loss/gain calculations, including a typology of currencies, considerations when selecting reference (or benchmark) conditions, and sources of uncertainty regarding the achievement of NNL and some responses to addressing these. The intended audience for this document is ecological specialists and technical consultants advising companies, governments and/or others wanting to undertake a biodiversity offset.