This book will explain how group theory underpins some of the key features of particle physics. It will examine symmetries and conservation laws in quantum mechanics and relate these to groups of transformations. Group theory provides the language for describing how particles (and in particular, their quantum numbers) combine. This provides understanding of hadronic physics as well as physics beyond the Standard Model. The symmetries of the Standard Model associated with the Electroweak and Strong (QCD) forces are described by the groups U(1), SU(2) and SU(3). The properties of these groups will be examined and the relevance to particle physics will be discussed.