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An endoscope is an instrument that brings illumination and imaging into a region that is difficult to view directly. Optical designs can use a relay-lens train, an ordered fibre bundle or a camera sensor near the tip.

An ordered image bundle must preserve relative fibre positions to transmit an image; a randomly arranged illumination bundle need not. Resolution, viewing angle, illumination and contrast depend on the complete optical system. The geometry that makes internal access possible also constrains aperture and image quality.

Endoscopic ultrasonography combines an endoscope with an acoustic transducer to obtain ultrasonography images from within the gastrointestinal tract. Positioning the source and receiver near a target shortens the intervening acoustic path and can reveal the wall and nearby structures. A probe integrated into the endoscope tip and a probe passed through its working channel are different apparatus arrangements.^[ref]