Science Day, held at each session of the Scientific Groups of the London Convention and Protocol, this year addressed marine geoengineering (23 April). The symposium covered: Understanding Marine Geoengineering; Emerging issues in Marine Geoengineering; Research and commercial ventures - when will it be ready?; Impact assessment, monitoring of Marine geoengineering - How to do it?; and Lessons learned - what works...or does not?. Under the current regulatory regime for marine geoenginnering, as adopted in the 2013 amendments to the London Protocol, the only marine geoengineering activity permitted is ocean fertilization and this may only be considered for a permit if it is assessed as constituting legitimate scientific research.