This document is one of two Resource Papers (the other being on No Net Loss) written to update information published in the BBOP Offset Design Handbook (2009) and to support the interpretation and understanding of the Principles, and of the Criteria and Indicators developed for the BBOP Standard on Biodiversity Offsets. The document specifically addresses Principle 2: ‘There are limits to what can be offset’. The paper outlines a set of ecological and other factors that can help to determine whether impacts are likely to be easy or difficult to offset – broadly arranged according to a green-amber-red system of categories which correspond to the likely level of risk involved with proposing an offset in a particular situation. It then suggests and describes the kind of evidence (‘verifiers’) that should be produced to demonstrate the offsetability of impacts for each category. The intended audience for this document is ecological specialists and technical consultants advising companies, governments and/or others wanting to undertake a biodiversity offset.