Photocatalysis has cemented itself as an important tool for synthetic organic chemists, allowing for thermally inaccessible transformations to be conducted at room temperature utilising visible light energy as the driving force. During a photocatalytic reaction, the photocatalyst (PC) is excited into an excited state (PC*) by absorbing incident visible light. After rapid internal conversion processes the PC* can subsequently interact with substrates (S) in several different ways. If the PC* transfers its energy to S, the activation mode is termed photoinduced energy transfer (PEnT), and can occur via either a Dexter or a Förster energy transfer mechanism. If the interaction involves single-electron transfer, the mechanism is termed photoinduced electron transfer (PET), occurring via oxidative quenching (where the PC* is initially oxidised to PC +• and S is reduced to S -• ) or reductive quenching (where the PC* is reduced to PC -• and S is oxidised to S +• ) catalytic cycles. Collectively, PET photocatalalysis is commonly referred to as photoredox catalysis.