Necrosomes formed by RIPK1–RIPK3 mediate necroptosis. Super-resolution microscopy identifies the architectural features of necrosomes and provides mechanistic insights into the signalling from RIPK1 to RIPK3 when RIPK1 is activated to mediate necroptosis, and from RIPK3 to RIPK1 when RIPK3 is inhibited to mediate apoptosis.