Polymeric materials are encountered everywhere: rubber, plastics, paint, soft contact lenses; the list is endless. There are two basic types of polymer, linear and non-linear. The linear polymers can be envisioned as a mass of long spaghetti-like strands. The molecules of non-linear polymers have a branched tree-like structure and can form ‘infinite’ networks. The basic building blocks of polymers are monomers. Monomers have reactive sites: chemical bonds can form between sites on different monomers leading to the formation of many monomers into a large molecule (polymer). If all monomers have one or two reactive sites, linear polymers result. If some of the monomers have three or more reactive sites, a non-linear (branched) molecule results. For many materials the reaction between monomers can be modelled as a random process; thus probability theory is the natural tool for studying polymers.