Despite the difficulties associated with controlling the polymerization of ring-strained nitrogen containing monomers, the resulting polymers have many important applications, such as antibacterial and antimicrobial coatings, CO₂ adsorption, chelation and materials templating, and non-viral gene transfection. This review highlights the recent advances on the polymerizations of aziridine and azetidine. It provides an overview of the different routes to produce polyamines, from aziridine and azetidine, with various structures (i.e. branched vs. linear) and degrees of control. We summarize monomer preparation for cationic, anionic and other polymerization mechanisms. This comprehensive review on the polymerization of aziridine and azetidine monomers will provide a basis for the development of future macromolecular architectures using these relatively exotic monomers.