The Book of Abstracts of ISOM XXI, the 21st International Symposium on Olefin Metathesis and Related Chemistry, contains short sketches of the content of 6 plenary lectures, 26 invited lectures, 24 contributed lectures and 59 poster presentations. Topics cover catalyst development and Organometallic Chemistry of carbenes and carbynes, the application of Olefin Metathesis and related chemistries in Organic, Polymer, Materials and Pharmaceutical Chemistry as well as in sustainable use of renewable resources.