The present dissertation comprises four empirical studies that examine different aspects of the phenomena of fears, phobias and anxieties in children and adolescents, at different levels of application. At the same time, this dissertation corresponds to the first randomized treatment study carried out with children suffering from anxiety disorders in Sweden. The first study examined fearfulness and its relation to general anxiety in 550 Swedish school children. It was found that Swedish school children report similar fears to those reported in other Western countries, but that Swedish girls report significantly lower fear levels than girls in other Western countries. In the second study, the relationship between phobic children's levels of fears and anxieties and those of their parents was examined. The children's levels of fears and anxieties were not found to correlate with those of their parents, nor with the parents' expression of fear. The third study compared two forms (child treated alone vs. parent present) of one-session exposure treatment with no treatment for children suffering from a specific phobia. It was found that one-session exposure treatment has significantly better results than no treatment but that the presence of one of the children's parents during treatment does not affect the treatment outcome. The fourth study examined how the children treated with the one-session exposure treatment had experienced the treatment. Based on the children's reports, it was concluded that the children experienced the treatment positively and did not consider it stressful or negative, despite the short duration of the treatment. Finally, in the general discussion the potential benefits of the present study are suggested, as well as new areas of research.