The design and synthesis of molecular containers is playing an increasing role in the selective removal and detection of species within solution. The cavities offered by such species provide the possibility of three-dimensional molecular recognition and therefore highly selective host species. Many varied approaches towards the design of container compounds have been adopted, ranging from rigid, covalently formed carcerands to self-assembling dimers and oligomers. This chapter explores the wide range of approaches possible; covalently formed containers, cages assembled around metal centres and those which self-assemble via non-covalent interactions. The main uses of such systems, for stabilising reactive species and promoting reactions within the protective environment of cavities, are also highlighted.