Chapter 4 will discuss the changing policy attitudes facing nuclear energy once it is discussed in a security of supply perspective. The reason is that the issue depends on subjective value-judgements, which may vary across market actors, citizens and countries. Markets thus cannot develop the verifiable parameters over which private buyers and sellers could negotiate satisfactory outcomes at sufficiently low cost. This informational complexity -even though it is due to very different reasons - is something energy supply security has in common with certain types of environmental externalities such as greenhouse gas emissions, the other great example of a market failure in the energy field.