Abstract
Quantum halos are defined as systems with dominating few-body structure and radii large compared to the sizes of the classically allowed regions. Dimensionless, universal scaling plots of radii vs. binding energies of two- and three-body systems are constructed to characterize and classify halo states. Short-range as well as long-range interactions are considered and molecular, atomic and nuclear states appear on the same plot. Specific two- and three-body systems are discussed, including Efimov states.