This is the first volume of the new series Key Concepts of Chess. The idea is to deal with middlegames in which a certain structure or essential theme becomes central. The idea of starting this series is in line with the Understanding before Moving series because the aim, is to improve the understanding of club chess players. According to the author, playing an opening should be 'hung up' on stereotypical ideas and concepts belonging to this structure. The thorough exploration of middlegame structures is beyond the scope of opening books, hence it makes more sense to expose it in this complementary series. In this first volume, the Hedgehog System is discussed in detail. Apart from being part of the author's repertoire, this typical set-up deserves to be widely highlighted. Although in the second part of the trilogy on the Sicilian, the Hedgehog has already made its appearance, in this book the seemingly fragile, but oh so treacherous creature can reconsider its spines in an extensive way.