David Howman, Director General, World Anti-Doping Agency

Whilst watching cycling's greatest race, the Tour de France, are you admiring the remarkable strength and stamina of the riders, or privately wondering whether their performance is fuelled by illegal substances? So much elite sport is tainted by our knowledge of past cheating.

Stephen Sackur talks to David Howman, director general of the World Anti-Doping Agency, and asks why it is proving so hard to root drugs out of sport.