This research article reports the cryo-electron microscopy structures of the prefusion and postfusion conformations of the SARS-CoV-2 spike protein (S protein), which is crucial for viral entry into host cells. The prefusion trimer has three receptor-binding domains clamped by a segment near the fusion peptide, while the postfusion structure is decorated with N-linked glycans. The spontaneous transition to the postfusion state is independent of target cells. The study reveals that the prefusion trimer is more tightly packed than previously observed stabilized soluble trimer structures, and the postfusion S2 trimer is highly stable and rigid. The presence of N-linked glycans on the postfusion S2 trimer suggests potential protective roles against host immune responses and harsh external conditions. These findings enhance our understanding of SARS-CoV-2 entry and may guide the development of vaccines and therapeutics. The article also discusses the implications for vaccine design, including the potential exposure of immunodominant epitopes and the stability of prefusion-stabilizing mutations.This research article reports the cryo-electron microscopy structures of the prefusion and postfusion conformations of the SARS-CoV-2 spike protein (S protein), which is crucial for viral entry into host cells. The prefusion trimer has three receptor-binding domains clamped by a segment near the fusion peptide, while the postfusion structure is decorated with N-linked glycans. The spontaneous transition to the postfusion state is independent of target cells. The study reveals that the prefusion trimer is more tightly packed than previously observed stabilized soluble trimer structures, and the postfusion S2 trimer is highly stable and rigid. The presence of N-linked glycans on the postfusion S2 trimer suggests potential protective roles against host immune responses and harsh external conditions. These findings enhance our understanding of SARS-CoV-2 entry and may guide the development of vaccines and therapeutics. The article also discusses the implications for vaccine design, including the potential exposure of immunodominant epitopes and the stability of prefusion-stabilizing mutations.