To support approval of pharmaceuticals for long term use in humans it is required that product safety is supported by acute and chronic toxicity studies in rodents and non-rodents. The duration of acute toxicity studies (S4A) and chronic rodent studies (S4B) were harmonised between the three ICH regions in 1991; whereas the process of harmonising the duration of non-rodent studies was initiated. The US FDA originally required studies of at least 12 months duration, whereas in Japan and EU studies of 6 months duration were considered acceptable as an ICH objective. In this chapter, the background for the ICH S4B guideline regarding the duration of non-rodent repeated dose toxicity studies is explained and lessons learned are discussed. Since the guideline was issued in 1997 changes occurred in e.g. the language of the European legislation, and the requirements for non-clinical studies to support clinical development has progressed within the ICH (M3): we therefore consider options such as prospective evaluation, biomarker-based mechanistic understanding, toxicokinetics and the use of evidence-based medicine to support further joint activities to harmonise the duration of non-rodent toxicity studies at the global level.