The reflection group H_4 encodes the symmetries of the regular polytope called the 600-cell, which is a four-dimensional analogue of the icosahedron. The structure of H_4 has many interesting relations to a variety of topics of mathematics and physics. This book outlines the ideas behind the classification of finite reflection groups, giving a context for H_4 as the largest exceptional noncrystallographic reflection group. Different constructions of H_4 and its root system, which have thus far been published in pieces throughout the mathematics literature, are then described and compared. The...