This book is based on a series of lectures taught by the author to all incoming first year Oxford University postgraduates in experimental particle physics. It begins by deriving the Dirac equation and incorporating the electro-magnetic interaction and calculating several “bread and butter” processes at tree level using the Feynman Stueckelberg approach: Mott scattering, electron–electron scattering, electron–positron scattering, Compton scattering, Bremsstrahlung and electron–positron to muon–anti-muon. The intention is for the student to become fluent in detail with all the steps leading to the calculation of these processes. Every step is motivated using the most basic arguments.