An endoscope is the instrument used for examining the inside of the body — a thin tube, rigid or flexible, carrying a light and a camera to its tip. Threaded through the mouth or the rectum, it lets gastroenterologists see the oesophagus, stomach, duodenum, and colon directly: finding ulcers, coeliac damage, tumours, and sources of bleeding, and taking biopsies or removing polyps through its working channel. The flexible video endoscope replaced the old rigid scopes and fibre optics, giving high-definition views and therapeutic capability in one instrument. For nutrition practice, endoscopy provides the ground truth — the villous atrophy of coeliac disease, the erosions of reflux — against which dietary treatment is judged. It turned the gut from a dark mystery into an inspectable organ.