Intendancy for sustainability has made it necessary for the highways industry to adapt its traditional processes to more cost-effective, energy efficient and greener technologies. This research programme was developed with a key aim of investigating the technical viability of aggregates formed with different combinations of Recycled Concrete Aggregate (RCA), Reclaimed Asphalt Pavement (RAP) and Natural Aggregates (NA) when used in construction of unbound subbases of highway pavements. As strongly evident from a comprehensive literature review carried out, little information is available on the application of RCA and RCA/RAP as subbase materials. The suitability of RCA blended with RAP and NA were investigated and compared to the British Standards, Highway Agency specifications and the AASHTO standards requirements for highway design. Having established their suitability for highway design, then the performance of the materials was assessed under traffic loading. In compliance with the requirements of the above Standards/Specifications the in-situ loaded behavior of the aforementioned materials were also investigated. This was achieved through numerical analysis of a typical pavement structures comprising subbases made of the above materials. As a consequence, a series of analyses were carried out using KENLAYER TM computer program to model the stresses and deformations in the subbases. The results of the analyses were then applied to the Mohr Coulomb failure model in order to predict the factor of safety against failure of the subbase layer. Amongst the salient findings from the research was that most of the materials tested complied with the standard requirements. From a large number of tests carried out on materials collected in the UK and Iran, it was also demonstrated that the source of a recycled material and the method of extraction had a significant influence on the engineering properties of the material, especially the CBR. In terms of compaction and CBR requirements, the 50%RAP+50%RCA mix was demonstrated as suitable for unbound subbase application. Also, the presence of RAP in the mixes of RAP/RCA was found to improve the drainage properties of an unbound subbase layer. From the viewpoint of durability and frost susceptibility, it was verified that all the materials investigated were applicable to the highest significance level (as defined by AASHTO). However, based on stiffness considerations, the materials were found to be applicable only to the lowest significance level. Now, turning to the toughness and shear strength properties, the same materials were found to fall in the middle significance levels. The results of KENLAYER TM modeling indicated that the safety factor against failure of the mixes containing RAP and RCA decreased as the stiffness decreased. Evidently the research is likely to have a number of implications on the design and construction of highways. Firstly, the demonstrated viability of mixes containing recycled materials (RCA+RAP) can lead to significant cost savings, reduced CO2 emissions, reduced exploitation of virgin materials and minimization of dumping of civil engineering waste. If embraced in industry, the suggested use of recycled materials can help support National and international targets regarding sustainability, environmental and energy conservation. The research succeeded in cataloguing the specifications of RCA/RAP and RCA/NA for practical works, followed by development and evaluation of different mixes of recycled aggregates obtained from different sources. Detailed assessment based on compliance with requirements, performance prediction and modeling was completed.