School of Physics, University of Sydney, NSW 2006, Australia
This year marks the 50th anniversary of the first papers on two separate types of coherent emission processes in space plasmas: Ginzburg and Zheleznyakov (1958) presented the first theory for what is now called plasma emission, and Twiss (1958) presented the first theory for what is now called electron cyclotron maser emission (ECME). In the first part of this talk, I will review the subsequent development of these theories, emphasizing the applications where they have been most successful. These are to solar and interplanetary type III bursts, and to Jupiterís decametric radio bursts (DAM) and the Earthís auroral kilometric radiation (AKR), respectively. In the second part of the talk, I will concentrate of the unresolved problems, emphasizing aspects of the observations that are not explained adequately by theory, or seem inconsistent with theory.