Camalexin (3-thiazol-2'-yl-indole) is the characteristic phytoalexin of Arabidopsis thaliana, which is induced by a great variety of plant pathogens. While particular pathogens, as well as a human tumour cell line, were growth inhibited by camalexin, some fungi show resistance due to active degradation. Camalexin originates from tryptophan and its biosynthesis involves the cytochrome P450 enzymes CYP79B2 and CYP71B15 (PAD3). Camalexin induction is a complex process, for which triggering by reactive oxygen species (ROS), salicylic acid signalling, and the glutathione status are important. Targets of the signalling cascade are the tryptophan and camalexin biosynthetic genes, which are strongly transcriptionally upregulated at the sites of pathogen infection. The important knowledge on camalexin, which is reviewed in this paper, will help to establish camalexin as a model for the investigation of the significance of phytoalexins in response pathogen challenge.