The colonoscopy is considered one of the most effective preventive examinations of all — and yet many people put it off for years. However, the colonoscopy is neither as uncomfortable nor as complicated as is often assumed. Anyone who knows the procedure and understands the central role played by the large intestine, the gut microbiome, and the early detection of polyps quickly realizes: this examination can be crucial in preventing colorectal cancer. This article explains in an easy-to-understand way how the large intestine works, how the preparation and examination proceed, and why regular prevention is so effective.