Endoscopy in humans is a powerful method for physicians to examine the gut for inflammatory or neoplastic changes. In medical and immunological research, animal models of intestinal diseases are established key tools to investigate the mucosal immune system, colitis and cancer development in the gut. Moreover, such models represent valid systems for testing of novel drugs. In the past, mice had to be killed in order to analyze colitis activity and tumor development. The following protocol describes a method to perform high resolution endoscopic monitoring of live mice. Mice developing colitis or colonic tumors are anesthetized and examined with a miniendoscope. The endoscope is introduced via the anus and the colon is carefully insufflated with an air pump. Endoscopic pictures obtained are of high quality and allow the monitoring and grading of tumors and inflammation. In addition, colonic biopsies can be taken. This protocol can be completed within 1 h.