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Studies of insect herbivory have mostly focused on leaf-feeding even though most woody plant biomass is stem tissue. Attack to stems has the potential to be more detrimental to plant performance than attack to leaves. Here we asked how severe is the impact of insect stem herbivory on plant performance. We quantify the effect of insect stem herbivory via a meta-analysis of 119 papers in 100 studies (papers by the same authors were treated as the same study). These studies involved 92 plant species and 70 species of insect herbivore (including simulated herbivory). Attack to plant stems reduced plant performance by an average of approximately 22%. Stem herbivory had greatest impacts on plant and branch survival, which was reduced by 63%. Measures of plant reproduction and vegetative biomass were reduced by 33% and 16% respectively, while measurements of photosynthetic rate were not significantly different between plants with and without stem herbivore attack. Stem herbivory led to a decline in leader performance but an increase in performance of laterals, highlighting the importance of plant compensation. Juvenile plants were more severely affected by stem herbivory than adult plants, and studies conducted in greenhouses found more severe effects than studies conducted in the field. Stem herbivory did not have a significant effects on any of the non-performance responses measured (defence compounds, SLA, root:shoot, phenology and plant carbon and nitrogen). We compare our results with results from various meta-analyses considering herbivory on other plant parts. The impact of insect herbivory to stems on plant performance appears at least as severe as insect herbivory to roots and leaves, if not more.