The mass of the electron is best determined with hydrogen-like ions in|a Penning trap. Binding in an ion removes the error due to the thermal|motion of the electron. However, the price is that some properties of|the electron are modified by the binding. These changes can be|computed in QED but there are interesting complications due to the|binding. I will present a new determination of the g-factor of a bound|electron. This calculation is motivated by the 2014 improvement in the|mass of the electron and makes further experimental progress|possible. Bound-state studies have traditionally been a mix of art and|science, and part of our motivation is to make them more algorithmic.