Martin Rees, Baron Rees of Ludlow, is a British cosmologist and astrophysicist. He was appointed the fifteenth Astronomer Royal in 1995. From 2004 to 2012, he was Master of Trinity College, Cambridge, and was President of the Royal Society between 2005 and 2010. He is co-founder of the Centre for the Study of Existential Risks at Cambridge University (CSER), where he is based as a Fellow at Trinity College.