Nanoengineered polymer carriers assembled by the layer-by-layer technique are being increasingly investigated as nano-to millimeter-sized, semi-permeable reactors. The reactors are assembled by the sequential adsorption of polymers that interact primarily via electrostatic forces, hydrogen bonding, or covalent bond formation onto a sacrificial colloidal template. Controlled permeability of molecular species is key to the functioning of these reactors and a number of techniques have been developed for measuring and controlling their permeability to both small molecules and larger macromolecules. The encapsulation of enzyme "machinery" into the carriers has produced a number of reactor examples capable of small molecule conversion. Advanced assembly techniques have been used to generate reactors with relevance to biomedicine, including biosensing, controlled drug release, and biopolymer synthesis.