Understanding the structure–property relationship in semiconducting nanostructures is key to tailoring the technologically important physical properties and can lead to the design of new nanostructured materials. Examples include nanostructures in optoelectronic devices ranging from solar cells, novel lasers, and optical amplifiers to physical representations of a quantum bit, or single polarized photon sources emitting quantum bits. The complexity of the problem is reflected in the fact that these nano-systems contain >10⁴ atoms, suggesting an extremely large phase space of sizes, shapes, and constituent materials. In this paper, we will (i) show how to establish the relationship between structure and properties in nanostructures; (ii) discuss the role of theory in the design of nanostructures with targeted physical properties, including the review and limitations of typically employed methods for electronic structure calculations; (iii) discuss possibilities to use predictive theory to “guide” nanostructures' fabrication; and (iv) address application of nanostructures for renewable energy issues and single-photon sources.