The scope and individual chapters of this book provide an indication of the importance of sea ice over a wide range of scales in time and space. Considering the degree to which ice cover is intertwined with other phenomena and processes, this brief introduction and overview can only skim the surface and cover selected aspects but not do justice to the subject matter as a whole. For more complete and rigorous treatises of the growth, structure and properties of sea ice, readers are referred to the overview articles by Weeks & Ackley 1986), Weeks 1998) and Wettlaufer 1998) in the comprehensive monograph by LeppaÈranta 1998). The same monograph also includes a highly relevant chapter on the optical properties of sea ice by Perovich 1998). Aspects of sea ice characteristics relevant for microwave remote sensing are covered by Hallikainen & Winebrenner 1992) and Tucker et al. 1992) as well as the paper by Golden et al. 1998b) which introduces a series of studies of sea ice electromagnetic properties. While Maykut 1986) provides a thorough treatment of ice growth and properties in the context of the sea ice energy and mass balance, Steele & Flato 2000) extend this to include a detailed discussion of ice growth models. A useful summary and review of sea ice physical property data sets and measurements has been provided by Yen et al. 1991). Articles by Pfirman et al. 1990) and Reimnitz et al. 1992) provide further insight into the importance of sea ice as a geological agent.