This chapter provides a brief history of the conceptual framework of plankton evolutionary ecology to explain why the role of defense in the evolution of unicellular plankton has been neglected so far. The aim of this historical overview is also to point out what can be learned from studying the arms race: the other side of the coin. Given the range of attack techniques to which protists are exposed-from viruses to zooplankton-armor is defined as all forms of mechanical defense against pathogens, parasites, and ingestors. These in turn will range from slimy or tough cell walls that hamper purchase, bar entry, or withstand puncturing to long spines that deter ingestion. Following a brief comparison with terrestrial systems the chapter expands on the argument, that evolution of eukaryotes in the plankton is driven by the arms race. To this end, the range of attack systems evolved by pathogens, parasites, and predators in the plankton is surveyed, align them with their respective defense systems, and speculate on their evolutionary history. In the final section, the chapter examines how the various types of armor, that is, cell walls, scales, frustules, and colony skin of selected groups provide protection against specific forms of attack. In order to link structural features of an organism to a function, knowledge of the physical and ecological context, andthus,of the factors that cause significant selection pressure, is crucial. © 2007 Elsevier Inc. All rights reserved.