This book introduces the reader to quantum groups, focusing on the simplest ones, namely the closed subgroups of the free unitary group.Although such quantum groups are quite easy to understand mathematically, interesting examples abound, including all classical Lie groups, their free versions, half-liberations, other intermediate liberations, anticommutation twists, the duals of finitely generated discrete groups, quantum permutation groups, quantum reflection groups, quantum symmetry groups of finite graphs, and more.The book is written in textbook style, with its contents roughly covering...