Nuclear Fusion, the ultimate sustainable energy source: clean, safe, for all and for ever. But exceedingly difficult to realise. Take this tour of the scientific principles and technologies of fusion energy and become familiar with its promises as well as its challenges. On the back of an envelope: little math, but common sense in spades. Oh, and there is the excursion to an international fusion research center such as ITER (Cadarache, France) or Wendelstein 7X (Greifswald, Germany), too.