The quantum state of a multi-particle system is said to be entangled if the wave function cannot be factored into a product of single-particle wave functions. Entanglement is one of the most distinctive features of quantum mechanics and gives rise to “ghostly” long-range correlations that may seem at first to violate special relativity. One dramatic example of this is the so-called Einstein-Podolsky-Rosen (EPR) paradox. There is no actual paradox, however, when one uses and interprets quantum mechanics correctly. Entanglement also arises from symmetry requirements imposed by quantum statistics as, for example, the “bunching” of photons and “antibunching” of electrons. The observability of such phenomena depends on the degeneracy parameter of the source. The author discusses the properties, in particular the degeneracy parameter, of the principal kinds of electron sources. Newly devised single-atom electron emitters should produce particle beams of sufficient degeneracy to make possible the novel correlation experiments proposed by the author.