Structure and light are both indispensable and interdependent elements of architecture. After acknowledging Louis Kahn's innovative integration of structure and light, this chapter explored how open structure can act as a source for light to enter a building. Structural form, members and even structural connections all participate in this role. The integration of structure and both transparency and the ingress of daylight is achieved by a variety of approaches. These include detailing structure with more small rather than fewer large members, penetrating solid structural members, and using glass or other translucent materials. Since sunlight is unwelcome in certain spaces, structure plays light-modifying roles. Structure filters and reflects, producing even and diffuse qualities of light. The chapter explores how light modifies our perception of structure. Light dematerializes structure, has structure read primarily as a source of light, and subvert awareness of structural rationality.