Various technological advances have made imaging an increasingly useful tool in the life sciences. Imaging techniques that move away from the limitations of wide-field microscopy have allowed deeper, higher-resolution imaging of thick biological tissue. Even within wide-field microscopy, advancements such as structured and sheet illumination, as well as improvements of biological probes, have led to better visualization of cells and their subcellular structures. The illumination source for wide-field microscopy, however, has remained relatively unchanged for decades, relying mainly on xenon, mercury, and halogen lamps. Light-emitting diodes (LEDs) have existed for more than 80 years, but their spectral range and output light flux have only recently become large enough for use in biological applications. This article presents the basic information necessary to build and to use an LED-based illumination source for microscopy. It also provides some useful resources about LED advancements. Although commercial LED-based illuminators for microscopy are available, custombuilt illumination systems can incorporate the latest LED chips into microscopes more quickly and inexpensively than is possible through the retail market.