SQIsign is the leading digital signature protocol based on isogenies and the only isogeny-based construction in the NIST standardisation process. In this talk, we introduce the SQIsign protocol, presenting its design and its connections to the most fundamental isogeny problems. We also discuss the many variants of SQIsign that have appeared over the last few years, with a particular focus on the recent changes that have become the round-2 NIST submission. Lastly, we discuss some recent results that provide a formal and complete proof of security of SQIsign, which fills a long-standing gap in the literature.