There is currently increasing interest in making a transition towards a circular economy, to improve resource efficiency and to reduce harmful emissions to the environment. The European Union adopted a Circular Economy package in 2015, while the Dutch government introduced a programme to achieve a circular economy by 2050. The construction industry is regarded in particular, being responsible for the use of about 50% of all raw materials. Innovation in the field of demountable and reusable structural elements is therefore of importance. By connecting composite (steel-concrete) slabs to steel beams using shear studs, composite beams are created. This results in a structurally efficient composite flooring system. However, demounting and reusing this system is very difficult due to the welded connection of the shear studs to the steel beam and the embedment of the studs in the concrete. This research aims to provide recommendations for the design of a demountable composite flooring system in which both the steel beams and the composite slab can be reused, while its structural efficiency is retained. Additionally, the environmental benefits of this system are assessed. To quantify the environmental advantages of a composite flooring system, a Life Cycle Assessment (LCA) has been conducted based on a case study of the Temporary Courthouse building in Amsterdam. For this building, it is found that the use of a composite flooring system instead of hollow core slabs leads to a reduction in environmental impact of 16-37%. For a building with main spans of 16.2 m instead of the original 10.8 m, this reduction increases to 35-51%. This is mainly caused by the reduction in the amount of concrete in the composite slab. Additionally, the weight reduction of 40-50% compared to hollow core slabs is beneficial for the transport-related environmental impact. Furthermore, it is found that the importance of the steel beams for the total environmental impact of the composite flooring system is limited: a reduction in steel section size due to shear interaction between the beam and the slab only leads to a reduction of 2-6% in environmental impact. However, a cost analysis shows that material costs are reduced with €12 - €31 per square metre when shear interaction is achieved. This shows that the use of demountable shear connectors between the beam and the slab can be viable, as long as the costs of the shear connectors are kept below these amounts. The structural behaviour of a composite beam with a composite slab and M20 grade 8.8 bolts as demountable shear connectors has been analyzed in more detail by means of analytical calculations and finite element analysis. It is found that initial slip due to bolt-to-hole clearances must be prevented in order to avoid larger deflections than allowed. A demountable composite flooring system is proposed in which the slab is cast in-situ for the first use, after placing the shear connectors, and reused as prefab elements without the shear connectors. In this way, bolt-to-hole clearances are avoided, while the main advantages of composite slabs are retained. Design recommendations are provided as a framework for the future development of a demountable composite flooring system.