Of the advanced fragmentation methods discussed in this book, electron-based techniques are the most widely adopted in practice. Meanwhile, of the four major classes of biomolecules, i.e., nucleic acids, proteins, carbohydrates, and lipids, the most commonly studied analytes by mass spectrometry are proteins. As the mechanism of electron capture (ECD) and electron transfer dissociation (ETD) is still debated, we will first consider the different proposals that have appeared in the literature over the years. Next, we will look at how practical implementations of these methods have evolved, from the initial inefficient production of photoelectrons to highly efficient modular designs that can be installed on a range of mass spectrometers today.