The control over comonomer sequences is barely studied in macromolecular science nowadays. This is an astonishing situation, taking into account that sequence-defined polymers such as nucleic acids and proteins are key components of the living world. In fact, fascinating biological machines such as enzymes, transport proteins, cytochromes or sensory receptors would certainly not exist if evolution had not favored chemical pathways for controlling chirality and sequences. Thus, it seems obvious that synthetic polymers with controlled monomer sequences have an enormous role to play in the materials science of the next centuries. The goal of this tutorial review is to shed light on this highly important but embryonic field of research. Both biological and synthetic mechanisms for controlling sequences in polymerization processes are critically discussed herein. This state-of-the-art overview may serve as a source of inspiration for the development of new generations of synthetic macromolecules.