Chain of custody (CoC) is one of the key system elements of most sustainability standards and voluntary certification schemes (referred to throughout this document as ‘sustainability systems’). By defining a set of requirements that control the movement, handling and processing of certified materials and volumes within and between certified companies (usually defined within a chain of custody standard), CoC enables the transfer of claims and communications about materials, products, processes and services covered by a sustainability system. An assurance system verifies whether certified companies have met relevant CoC requirements, underpinning trust in the claims made about tracked materials and attributes.