A conceptual framework for creating photonic crystals from self-assembling block copolymers has been formulated. In order to form useful band gaps in the visible regime, periodic dielectric structures made of typical block copolymers need to be modified to obtain appropriate characteristic distances and dielectric constants. moreover, the absorption and defect concentration must also be controlled. This affords the opportunity to tap into the large structural repertoire, the flexibility and intrinsic tunability that these self-assembled block copolymer systems offer. A symmetric poly(styrene-b-isoprene) (~400k M) block copolymer was used to achieve a photonic band gap in the visible regime. By swelling the diblock copolymer with lower molecular weight constituents, we have been able to control the location of the sotp band across the visible regime. one and three-dimensional crystals have been formed by changing the volume fraction of the swelling media. Methods for incorporating defects of prescribed dimensions into the self-assembled structures have been explored leading to the construction of a microcavity light-emiting device.