This article is the first in a three-part series whose overall aim is to more precisely define the terms 'additionality' and 'baseline' in the context of environmental policy and propose a conceptual framework for applying these concepts within offset programmes. Precise and theoretically well-grounded definitions of these terms is a necessary precursor to their application in real world offset programmes in a way that allows them to operate with both greater credibility and effectiveness. Definitions of 'additionality' and 'baseline' are proposed that can be applied to offset policies and programmes addressing public goods issues at any scale, from traditional project-based initiatives to those transcending traditional offset projects.