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This is a review paper concerned mainly with oscillations and fluctuations which have been observed to occur either spontaneously or as a result of deliberate excitation in low-pressure, hot-cathode plasmas. Experimental observations on high- and low-frequency effects occurring spontaneously are discussed, and speculations are made about the possible generation mechanisms. In many cases, theories of these mechanisms have been treated in experiments involving deliberate excitation of oscillations, and comparisons are made where relevant. For device applications, and to relieve the difficulties which occur in quantitative experiments in plasma physics, it is important that the sources of spontaneous oscillations be understood and suppressed, if possible. The paper includes a discussion of work which has been carried out to reduce these effects, and concludes with some suggestions for further lines of attack on the problems.