The electron is unique and is regarded as the smallest reagent. In addition, electron transfer is universally involved not only in chemical systems but also in many other systems such as electrical conduction and life activity. In the field of synthetic organic chemistry, development of novel electron injection methods has led to a new phase of chemical bond formation techniques. In addition to conventional metallic reagents and electrolytic techniques, synthetically valuable reactions with noble metal-free electron initiators driven by thermal or photochemical stimulus have recently been developed intensively. In particular, visible-light organic photoredox catalysis has rapidly spread among synthetic chemists as a game-changing strategy because of its sustainability and versatility. This review focuses on modern state-of-the-art electron injection strategies for organic synthesis.