This special issue is intended as a tribute to the work of Ralf Koetter. Ralf's premature death on February 2, 2009 deprived our community not only of one of its most beloved members, but also of an imaginative individual contributor, valued collaborator, and respected leader who had already had a significant impact on coding theory and practice, and who could have potentially contributed so much more. The contributions of Ralf Koetter to our field have been remarkably diverse and often seminal. They span a wide range of areas, from algebraic decoding algorithms, to codes on graphs, to signal processing, to network coding. This editorial provides an overviewof the most salient of his results. The overview is organized into four sections, presented below roughly in the order in which Ralf's interests developed during his career: Algebraic coding theory; Codes on graphs and iterative coding; Communications and signal processing; and Network coding.