In addition to water vapor, oxygen, and carbon dioxide, plants release a variety of volatile organic compounds (VOCs) from their vegetative organs. These volatiles can mediate plant interactions with the environment. For example, they can help plants defend themselves against biotic attackers, or they can act as signals within or between plants, eliciting defensive responses. Since volatiles are so frequently released by plants after herbivore damage, much effort has been made to elucidate the role of herbivore-induced volatiles. However, undamaged plants of many species emit VOCs too. In order to fully understand the function of VOCs, it is necessary to investigate the function of both induced and constitutive volatile emission. In this work, we studied the role of constitutive volatiles in plant direct (chapter I) and indirect defense (chapter II) in relation to herbivore-induced volatiles. Moreover, we investigated how volatile emission is affected by association with arbuscular mycorrhizal fungi (chapter III).