It is a pleasure to thank Sami Solanki and the editorial board of Living Reviews in Solar Physics for inviting me to write the present review article, and the editorial staff of Living Reviews for their professional help and their patience when unforeseen events took over. I am much indebted to my two referees, Jeffrey L. Linsky and Thierry Montmerle, whose thorough and critical reviews of this paper have improved both content and presentation. Special thanks go to Helmut Lammer, Yuri Kulikov, and Thomas Penz for providing me their new manuscripts before publication. Discussions with Tom Ayres and Helmut Lammer have stimulated inclusion of issues that I would otherwise have failed to mention. I thank the following colleagues for giving me permission to reproduce their original figures: Marc Audard, Tom Ayres, Max Camenzind, Alfred Glassgold, Christopher Johns-Krull, Eric Feigelson, James Kasting, Yuri Kulikov, Helmut Lammer, Antonietta Marino, Stephen Marsden, Sergio Messina, Thomas Penz, Ignasi Ribas, I.-Juliana Sackmann, Carolus Schrijver, Klaus Strassmeier, Alessandra Telleschi, and Brian Wood.
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