For decades, transition-metal catalysts based on noble metal have proven to be highly efficient for a wide range of organ-ic transformations. However, due to their low abundance and highly volatile price the use of such metal is now critical for the economy. Thus, the use of more sustainable alternatives is mandatory and the more abundant first-row transition metals are becoming interesting challengers. Among them low-valent 3d transition metals appeared as best candidates as they are intrinsically more reactive than high-valent ones. However, this reactivity implies that these complexes are often in-situ generated putting further away the aspect of atom economy and low waste. To circumvent this and to get more insight on the mechanism of reaction the concept of well-defined complexes has emerged. In this area, due to its historic development and the trouble associate with the preparation of other 3d transition metal well-defined complexes, cobalt appears as an important player. In this review, after a definition of what is a low-valent complex and the presentation of the concept of well-defined (pre)catalyst also known as the “single-component” strategy we will report the syntheses and applications in catalysis of low-valent well-defined cobalt complexes classify by their oxidation state and ligand environ-ment