Spectral tuning is a diverse topic, both with regard to mechanism and with regard to biological significance. We have touched upon a related topic already when dealing with quantum dots in Chapter 5. In organisms, spectral tuning can be achieved both by chemical means (choice of pigment) and by physical means. The latter aspect is treated towards the end of the chapter in a section on structural color. As for the functional aspect, spectral tuning has significance for photosynthesis, vision, bioluminescence, and coloration both for protection and signalling in various contexts.