I dedicate this work to the unfaltering support of my wife Lisa and my children, Isobel and Alistair.

My sincerest and heartfelt thanks go to the following people without whom this work would never have been completed:

• My supervisors Tim Bishop and Paul Finan for all their support and sound advice.
• The 'FlexiScope Girls', Angela Butler, Joyce Pearson and Sue Corcoran and the staff of the endoscopy unit at Leeds General Infirmary who were so enthusiastic and hard-working.
• Julie Miller, the FlexiScope trial administrator, who was invaluable and without her I am sure the organisation of the trial would have failed.
• Juliette Moore, Rupert Gaut and the other scientists and laboratory staff of the Cancer Genetics department of St. James's University Hospital for tolerating a surgeon in their midst.
• All the staff of the Cancer Research U.K. Genetic Epidemiology department at St. James's University Hospital for their help and support.
• Jenny Barrett, Faye Turner and Rob Edwards for their amazing ability to instil some statistical understanding to a simple medic.
• The FlexiScope Trial team headed by Wendy Atkin without whom there would have been no basis for this research.