As national geological disposal programmes progress towards implementation, the concept of “optimisation” and related requirements are receiving increased attention. Exchanges within NEA expert groups have shown that both regulators and implementers would benefit from a review of the relevant concepts and available guidance and experience. This report summarises and reviews the concepts relevant to the “optimisation” of geological disposal systems as they are outlined in national and international guidance. It also presents a set of observations and key questions. Overall, the report shows that, when addressing “optimisation”, there is ample scope for clarifying concepts, facts and possibilities and for ensuring that regulatory guidance is sufficiently precise and implementable. The intention is that this report should serve as a basis for discussion within and beyond NEA committees and expert groups.