Understanding the supply chain, customer behaviour and effect of offer/promotion in a large retail organisation such as M&S is a non-trivial task. Identifying the causal impact of one set of actions on another is an essential task in decision-making. In M&S, we use machine learning in conjunction with causal inference to answer the “why” and “what-if” questions. In this talk we will discuss samples of the use of causal inference and the application of Bayesian techniques in M&S.