 Depending on the particular health challenge we are concerned with, the public in question can be more local or more global than a single country’s population. National boundaries are relevant because individual countries usually set policies and regulations, which vary from country to country. They are also relevant for reasons having to do with government control: countries report their data about communicable disease outbreaks, the burdens of disease, and other health indicators to global institutions such as the WHO on a voluntary basis. Although International Health Regulations to which 196 countries (WHO 2014) are signatories provide an international structure for global public health, as with much international law and regulation, enforcement mechanisms are weak. The moral implications of these practical limitations for public health are profound. The structure of the problem is similar to environmental challenges such as climate change, which, as we note in 1.3, is itself increasingly being framed as a public health crisis. Some of the determinants of ill health are not restricted by national boundaries, nor can they be effectively addressed by any nation on its own.