The technique of Koehler Illumination is one of the most important and fundamental techniques in achieving optimum imaging in any given light microscope set-up. Although it should be routinely used as part of setting up a microscope, many microscopists are put off by thinking that the correct set-up is complex and time consuming and it is therefore still not widely practised. By getting to know the two main components of the microscope which are adjusted in this technique (the diaphragms and sub-stage condenser) in reality, correct set-up should only take a matter of minutes. A correctly aligned microscope can result in greatly improved images of uniform contrast and illumination as well as higher resolution and more detail. In this article, we will look at the history of the technique in addition to how to adjust the components in five easy steps.