Guanine crystals are widely used in nature to manipulate light. The first part of this feature article explores how organisms are able to construct an extraordinary array of optical “devices” including diffuse scatterers, broadband and narrowband reflectors, tunable photonic crystals, and image‐forming mirrors by varying the size, morphology, and arrangement of guanine crystals. The second part presents an overview of some of the properties of crystalline guanine to explain why this material is ideally suited for such optical applications. The high reflectivity of many natural optical systems ultimately derives from the fact that guanine crystals have an extremely high refractive index—a product of its anisotropic crystal structure comprised of densely stacked H‐bonded layers. In order to optimize their reflectivity, many organisms exert exquisite control over the crystal morphology, forming plate‐like single crystals in which the high refractive index face is preferentially expressed. Guanine‐based optics are used in a wide range of biological functions such as in camouflage, display, and vision, and exhibit a degree of versatility, tunability, and complexity that is difficult to incorporate into artificial devices using conventional engineering approaches. These biological systems could inspire the next generation of advanced optical materials. How are organisms able to construct and control diffuse scatterers in white spiders, broadband and narrowband reflectors in fish scales, tunable photonic crystals in chameleons and copepods, and image‐forming mirrors in scallop eyes? Just by varying the size, morphology, and arrangement of the guanine crystals in their cells.