Quantum chromodynamics, the theory of the strong interaction, is a field theory of quarks and gluons. When it was formulated, the existence of its basic ingredients was still unproven and controversial. While for the quarks the case had been settled by 1975, it remained open for the gluons until in 1979 experiments at the electron-positron collider PETRA at DESY in Hamburg led to a breakthrough. Peculiar final configurations of hadrons produced in the electron-positron annihilation process at high energies, so-called planar events and three-jet events, were discovered. In a close cooperation between experiment and theory they were unambiguously identified as signatures of the radiation of hard gluons by quarks (“hard gluon bremsstrahlung”), providing the first clear and direct observational evidence for the existence of the gluon and confirming crucial predictions of quantum chromodynamics.