David John Cameron MacKay, FRS FInstP FICE (born 22 April 1967), is the Regius Professor of Engineering in the Department of Engineering at the University of Cambridge and from 2009 to 2014 was Chief Scientific Adviser to the UK Department of Energy and Climate Change (DECC). Before being appointed to the DECC, MacKay was most well known as author of the book Sustainable Energy – Without the Hot Air.