Phosphorylation plays a central role in the dynamic regulation of the processing of the amyloid precursor protein (APP) and the production of amyloid-β (Aβ), one of the clinically most important factors that determine the onset of Alzheimer's disease (AD). This has led to the hypothesis that aberrant Aβ production associated with AD results from regulatory defects in signal transduction. However, conflicting findings have raised a debate over the identity of the signaling pathway that controls APP metabolism. Here, we demonstrate that activation of the c-Jun N-terminal protein kinase (JNK) is essential for mediating the apoptotic response of neurons to Aβ. Furthermore, we discovered that the functional loss of JNK signaling in neurons significantly decreased the number of amyloid plaques present in the brain of mice carrying familial AD-linked mutant genes. This correlated with a reduction in Aβ production. Biochemical analyses indicate that the phosphorylation of APP at threonine 668 by JNK is required for γ-mediated cleavage of the C-terminal fragment of APP produced by β-secretase. Overall, this study provides genetic evidence that JNK signaling is required for the formation of amyloid plaques in vivo. Therefore, inhibition of increased JNK activity associated with aging or with a pathological condition constitutes a potential strategy for the treatment of AD.