Electron capture supernovae (EC-SNe) are the deaths of approximately 8-10M stars. In this thesis, the evolution of 8-12M stars is calculated using the MESA stellar evolution code. The aims of this thesis are to produce the most advanced and up-to-date progenitor models for electron capture supernovae (EC-SNe) and to study the behaviour of stars across the transition mass range between AGB stars and massive stars. These new stellar models will be the first of their kind since the 1980s and highlight new computational successes and persisting challenges in the field of stellar physics.