Dafydd Rhys "Dave" Williams OC OOnt BSc MSc MD CM CCFP FCFP FRCPC FRCP DSc LLD is a Canadian physician, public speaker, CEO, author and a retired CSA astronaut. Williams was a mission specialist on two space shuttle missions. His first spaceflight, STS-90 in 1998, was a 16-day mission aboard Space Shuttle Columbia dedicated to neuroscience research. His second flight, STS-118 in August 2007, was flown by Space Shuttle Endeavour to the International Space Station. During that mission he performed three spacewalks, becoming the third Canadian to perform a spacewalk and setting a Canadian record for total number of spacewalks. These spacewalks combined for a total duration of 17 hours and 47 minutes.
In 1998, Williams became the first non-American to hold a senior management position within NASA, when he held the position of Director of the Space and Life Sciences Directorate at the Johnson Space Center and Deputy Associate Administrator of the Office of Spaceflight at NASA Headquarters.