Implementations of the tissue illumination systems, devices, and methods disclosed herein take advantage of the translucent nature of tissue to reveal properties by light transmission, for example, tissue type, tissue transition locations, underlying structures, and the like, that are not easily distinguished by reflected light. Illuminating a back-side of a translucent tissue permits a user to distinguish between different types of tissue, tissue transition locations, and/or structures that are difficult or impossible to discern under overhead or front-side illumination. Implementations include a light source that is positionable behind a tissue or disposable within a body cavity or duct, for example, within a heart ventricle.