Inspired by elegant polymerizations in biological systems, polymer synthesis in confined artificial nanospaces is a key challenge in the control of polymer structures and the design of well-defined nanostructures. In this regard, porous coordination polymers (PCPs) have a wide range of advantages, such as regular channel structures, controllable pore size, dynamic and flexible pores, and unique surface potentials and functionality, which can be utilized for precisely controlled polymerization and polymer arrangement. This Focus Review describes recent progress in polymerization in the nanochannels of PCPs and demonstrates why this polymerization system is so attractive and promising, from the viewpoints of three essential polymerization processes in PCPs, that is, monomer arrangement, polymerization methods, and control of polymer structure.