Abstract This chapter reviews the effects of alien terrestrial arthropods on the economy, society and environment in Europe. Many alien insect and mite species cause serious socio-economic hazards as pests of agriculture, horticulture, stored products and forestry. They may also affect human or animal health. Interestingly, there is relatively little information available on the exact yield and financial losses due to alien agricultural and forestry pests in Europe, particularly at continental scale. Several alien species may have a positive impact on the economy, for example parasitoids and predators introduced for the biological control of important pests. Invasive alien arthropods can also cause environmental hazards. They may affect native biodiversity through various mechanisms, including herbivory, predation, parasitism, competition for resource and space, or as vectors of diseases. They can also affect ecosystem services and processes through cascading effects. However, these ecological impacts are poorly studied, particularly in Europe, where only a handful cases have been reported.