Nonlinear behaviour of plasma due to resonant three-wave interactions is considered, both as a decay instability arising from a single pump wave and as resonant mixing when two waves are initially present in the plasma. Observations of both processes involving longitudinal electron (Langmuir) waves and ion waves are reported and quantitative measurements given. The interaction between coherent waves is related to that between random-phase waves and in this way the cross-section for plasmon-plasmon interaction derived. In a finite plasma, resonant three-wave interactions in which all waves are Langmuir waves are possible and quantitative results for this process are given. The new results of this paper and an earlier one are summarized and combined in relation to the dispersion diagram of Langmuir waves.