A common theme has run through the chapters of this book. How can intelligent systems master complex environments such as bao, chess or Go, where a combinatorial explosion typically forbids exhaustive search? A ﬁrst answer was oﬀered by our review of the state of the art in computer board-game playing. There, we saw that simple board games can be solved by exhaustive enumeration of all positions, while complex games such as Go still challenge technology and artiﬁcial intelligence. With several games of medium complexity, such as checkers or chess, computers’ success rely on a combination of brute search, selective search, eﬃcient evaluation functions, and substantial knowledge bases covering openings and endgames. An unexpected outcome of this rapid progress is that computers can now be used to explore aspects of human cognition.