As a result of increasing demand for primary aggregates, coupled with an escalating environmental awareness, the UK government is committed to promoting more efficient use of natural sources and secondary aggregates. Part of this strategy involves developing methodologies for enhancing the effective use of substitutes for primary aggregates, such as industrial waste, by-products and recycled materials. One such area is the use of recycled materials in sub-base layers. Previous TRL studies have led to the introduction of recycled asphalt arising as type 4 sub-base in the Specification for Highway Works. This report summaries TRL research carried out on these types of materials over the past tweleve years. In total, seven sub-base trials are reported, encompassing three initial specification development trials, two specification validation trials and small compaction trial. Data from the first full-scale site use of asphalt arising as Type 4 sub-base in the UK are also reported. Other relevant data and advice on the use of asphalt arisings in sub-base, capping and structural fill applications are also presented and discussed. It was concluded that Type 4 asphalt arising sub-base perform in a manner comparable to conventional Type 1 granular sub-base. The current specification requirements for both type 1 and Type 4 materials has been shown to be appropriate and no major amendments are proposed, although a 200 mm maximum compacted layer thickness is recommended for type 4 sub-base applications