Updated project metadata. Receptor interacting kinase 1 (RIPK1) regulates cell death and inflammatory responses downstream of TNFR1 and other receptors, and has been implicated in the pathogenesis of inflammatory and degenerative diseases. RIPK1 kinase activity induces cell death by activating apoptosis and necroptosis, however the mechanisms and phosphorylation events regulating RIPK1-dependent cell death signalling remain poorly understood. Here we show that RIPK1 auto-phosphorylation at serine (S) 166 plays a critical role for the activation of RIPK1 kinase-dependent apoptosis and necroptosis. Moreover, we show that S166 phosphorylation is required for RIPK1 kinase-dependent pathogenesis of inflammatory pathologies in vivo in four relevant mouse models. Mechanistically, we provide evidence that trans auto-phosphorylation at S166 modulates RIPK1 kinase activation but is not by itself sufficient for the induction of cell death. These results show that S166 auto-phosphorylation licences RIPK1 kinase activity to induce downstream cell death signalling and inflammation, suggesting that S166 phosphorylation can serve as a reliable biomarker for RIPK1 kinase-dependent pathologies.