The 20th century history of the particle concept is a story of disillusion. It turned out that in the subatomic domain there are no particles in the classical sense. Neither the atoms, nor their constituent parts, nor the causes of the particle tracks observed in the cloud chamber, nor the pointlike or extended scattering centers within some target matter are classical particles. And it will turn out in the course of this chapter that a generalized concept of quantum particles is not tenable either. Particles are experimental phenomena rather than fundamental entities.