The statistics of indistinguishable particles is among the most fundamental of physical effects. The Pauli exclusion principle -- that every occupied quantum state contains at most one fermion -- controls the formation of matter. The basic building blocks of matter such as protons, neutrons, and electrons are all fermions. Conversely, particles such as the photon, which mediate forces between matter particles, are all bosons. A spin-statistics theorem attempts explain the origin of this fundamental dichotomy.