Book excerpt: This book is devoted to an extensive and systematic study on unitary representations of the Poincar‚ group. The Poincar‚ group plays an important role in understanding the relativistic picture of particles in quantum mechanics. Complete knowledge of every free particle states and their behaviour can be obtained once all the unitary irreducible representations of the Poincar‚ group are found. It is a surprising fact that a simple framework such as the Poincar‚ group, when unified with quantum theory, fixes our possible picture of particles severely and without exception. In this connection, the theory of unitary representations of the Poincar‚ group provides a fundamental concept of relativistic quantum mechanics and field theory.