This item is a tutorial on the topic of oblique illumination in optical microscopy. Specimens that are nearly transparent may be almost invisible when viewed in traditional modes. If illumination is directed to strike the specimen from an oblique angle, details may be revealed with far greater contrast and clarity. The tutorial includes links to four related Java simulations. This item is part of a larger collection of materials on optics and microscopy developed by the National High Magnetic Field Laboratory and Florida State University.