A colonoscopy (/ˌkɒləˈnɒskəpi/) or coloscopy (/kəˈlɒskəpi/) is a medical procedure that uses an endoscope to examine the large intestine (colon) and the lower part of the small intestine. The procedure involves a flexible tube fitted with either a CCD or fiber optic camera, which is inserted through the anus to visualize the internal lining of the bowel.

The main purpose of a colonoscopy is to inspect the colon wall for abnormalities such as ulcers or precancerous polyps. It also allows doctors to perform biopsies or remove suspicious lesions that could indicate colorectal cancer.

While similar to a sigmoidoscopy, a colonoscopy examines the entire colon rather than just the sigmoid portion. The colon measures roughly 120–150 cm (4–5 feet), so a full colonoscopy offers a complete assessment, whereas a sigmoidoscopy only covers the distal 60 cm of the colon. This distinction is important because many of the survival benefits of colonoscopy screening come from detecting lesions in the distal colon.

Screening practices for colonoscopy vary worldwide. In the United States, it is a widely recommended method for colorectal cancer screening, typically starting between ages 45 and 50 depending on individual risk factors and guidelines from groups like the American Cancer Society. In contrast, many European countries rely more on fecal occult blood testing (FOBT) or sigmoidoscopy for population-based screening. These differences are influenced by healthcare systems, policy decisions, and cultural factors. Recent research emphasizes the importance of global awareness campaigns and effective screening strategies to reduce colorectal cancer rates.