1.1 Plant-Insect Interactions Insects are the most species-rich class of eukaryotes on earth and roughly half of all insect species are herbivores. Besides being species-rich, insects are also quite abundant, making up more biomass than any other animal class. It is therefore likely that during their life, most plants will encounter herbivorous insects chewing on their tissue, sucking up their cell content and/or passively feeding on their vascular sap. Not surprisingly, plants are not defenseless against these herbivores. Plants have evolved a wide range of defense mechanisms which can be constitutively present and/or induced by herbivory. Many of these defense mechanisms have a direct effect on the herbivore by negatively affecting its physiology (e.g. through toxins or anti-nutritional compounds) or by interfering with its behavior (e.g. through repelling or deterring compounds) (Schoonhoven et al., 1998). Besides direct defense mechanisms, plants can make use of ‘bodyguards’ to protect themselves against herbivorous insects. Often, these bodyguards are insects themselves: predators or parasitoids preying or parasitizing on the herbivores. Plants can reward these bodyguards by providing them shelter, additional food and/or information (Van Poecke and Dicke, 2004). But plants do not only rely on insects as bodyguards. For many plant species insects form an essential part of plant reproduction by transporting their pollen (Pichersky and Gershenzon, 2002). Clearly, insects play an important part in the ecology of plants, whether considered from a natural, agricultural or an evolutionary perspective. In the last decade, Arabidopsis thaliana has been introduced to studies on plant-insect interactions, especially with the aim to get a better understanding of the plant molecular mechanisms underlying this interaction. As the number of publications on Arabidopsis - insect interactions is steadily increasing, this review aims to summarize their most important findings.