Phenylene-based polymers are a widely studied class of blue-emitting materials for use in polymer light-emitting diodes intended for display applications. This review looks at the various types of such polymers that have been synthesised and the problems that have arisen in obtaining stable and efficient blue emission from them. The emission colour from phenylene-based polymers can be controlled by linking some or all of the phenylene units with short carbon or nitrogen bridges, and it has been found that by varying the amount and type of bridges pure blue emission can be obtained. Both aggregation of chains, and the formation of emissive defects cause instability of the emission colour and it is shown that this can be surmounted by appropriate choice of substituents and synthetic routes. In particular the formation of defects is promoted by hydrogens on the bridgeheads, and polymers with aryl substituents at the bridgeheads have been found to much more stable than those with alkyl substituents. High device efficiency can be obtained by incorporation of appropriate charge transporting substituents, which also may help with reducing problems with colour stability. It is shown that by correct design of the polymer structure and appropriate synthetic strategy it is possible to obtain efficient, stable, pure blue emission from phenylene- based polymers.