There is some confusion in what is meant by “bio-based”, “biodegradable” and/or “compostable” plastics. This confusion is not only by consumers – there are differences in use of the terms in law and policy. The term “bio-based” refers to plastics manufactured, entirely or partly, from biological source material. However, issues arise as non-biologically sourced additives are used, or products combine more than one material from different sources. “Biodegradable” is used in different ways. Can materials be described as intrinsically biodegradable or can this term only be used in the context of specific environmental conditions in which materials are placed? What is the role of standards? Definitions of this term are used in different ways by different stakeholders and confusion may result. “Compostable” also has potential for confusion. Policy makers often limit this to industrial composting, yet the term itself does not do this and consumers may be misled. This briefing explores the different uses of these terms and proposes definitions which aim to overcome the confusion and inconsistency. It also makes recommendations for their use consistent with the principles of better law making. Law and policy should not use the terms without defining them.