Yasen Nikolov's book studies figures of law, which rarely have been a subject of attention in the last twenty years in Bulgaria. Today they appear before us as "lifebuoys" in time of crisis. Time, which is straining the normal and consistent development of social and economic relations. The book is divided into two parts, the first devoted to the figure of clausula rebus sic stantibus, the second - to vis major (force majeure) They appear to be very close, follow the same legal and economic logic, and this requires addressing them in a single work. Their role is to find that balance between performance and justice, which is most useful to the society as a whole.