Research was conducted on the reuse of construction waste, particularly as aggregate for use in the subbase layer in road pavement construction. Although Britain is relatively rich in natural aggregate reserves, planning approvals to develop new quarries are running at about half the rate of extraction. This means that in the future the rate of production of natural aggregate will be too slow to meet the demands of the construction industry. The use of secondary materials is not likely to create a major source of aggregate, but if recycled aggregate was used in less demanding situations the need for natural aggregate would be reduced. The research focused on the properties of crushed concrete (aggregate obtained from the break-up, crushing, and screening of concrete slabs from road pavements) and demolition debris (aggregate obtained from the recycling of general demolition debris). The performance of these materials was compared, mainly on the basis of laboratory testing, with that of limestone, which is commonly used as subbase aggregate in Britain. The tests on the aggregates included California bearing ratio, compaction, and frost susceptibility tests. In most of the tests, the recycled aggregates performed as well as limestone; it is therefore concluded that recycled aggregates could be used as alternative subbase aggregates. However, some proper guidelines on the production and quality of recycled materials are needed.