The ICH S9 Guideline: Nonclinical Evaluation for Anticancer Pharmaceuticals reached Step 4 in November 2009 and the guideline was a significant advance in promoting anticancer drug development. Since reaching Step 4, all the parties using the guideline have experienced some challenges around implementation. Implementation of the guideline has revealed areas that are open to broad and divergent interpretation by both regulatory authorities and industry. For this reason, an Implementation Working Group (IWG) was formed in October, 2014, by the International Council for Harmonization, formerly the International Conference on Harmonisation (ICH), to develop Questions and Answers to provide additional clarity around anticancer pharmaceutical development. The Questions and Answers developed by the IWG are intended to facilitate the implementation of the S9 Guideline and, of additional benefit, to continue progress in the 3Rs of Reduction, Refinement, and Replacement in use of animals.