In this book the concept of indistinguishability is defined for identical particles by the symmetry of the state. It applies, therefore, to both the classical and the quantum framework. The author describes symmetric statistical operators and classifies these by means of extreme points. For the description of infinitely extendible interchangeable random variables de Finetti’s theorem is derived and generalizations covering the Poisson limit and the central limit are presented. A characterization and interpretation of the integral representations of classical photon states in quantum optics are derived in abelian subalgebras. Unextendible indistinguishable particles are analyzed in the context of nonclassical photon states. The book addresses mathematical physicists and philosophers of science.