This document provides guidance for ancillary material (AM) users. It is applicable to cellular therapeutic products, including those gene therapy products whereby cells form part of the final product. It does not apply to products without cells. This document focuses primarily on ancillary materials (AMs) of biological (human and animal) origin and their potential impurities and contaminants. NOTE 1 The decision chart in Figure 1 illustrates the rationale underlying the scope of this document. However, diverse biological sources, including plants, insects and marine organisms, can also be used in the development of a cellular therapeutic product. Therefore the fundamental principles of risk management also apply for these sources of AMs. This document does not cover the selection, assessment or control of starting materials and excipients. However, it is anticipated that these are still covered by general risk management procedures. This document is applicable for users at all stages of clinical development and supply; however maximum benefits can be gained by the implementation of the recommendations in the early stages of development. NOTE 2 International, regional or national regulations or requirements can also apply to specific topics covered in this document.