Molecular imprinting is a highly attractive approach for producing binding sites in polymeric materials. It can be described as a way of making artificial locks for molecular keys. With this technique, lock-building elements are assembled in the presence of a molecular key and subsequently are made transfixed by cross-linking. The prepared lock becomes selective for the chosen molecular key and will not effectively recognize other keys. The molecular key may, in principle, be any type of molecule—from small molecules such as drug substances, amino acids, or steroid hormones to large molecules such as nucleic acids or proteins. Molecular assemblies such as cells and viruses can also be perceived. The lock-building elements consist of different monomers capable of forming three-dimensional network polymers.