This project aims to model demand-side flexibility coming from aggregation of a large number of residential and small and medium-size commercial end-users in the distribution network (DN). The algorithms developed through this project will facilitate more flexible operation of the DN by assessing the time varying capacity available from flexible loads, in response to flexible services currently procured by the distribution system operator (DSO), namely: Sustain, Secure, Dynamic and Restore. The aggregate flexibility will be described as the amount of available capacity and its duration, as a result of aggregating individual loads with different operating modes, start times, maximum deferral times, etc., driven by the end-users' daily behaviour and constrained by their comfort. Such flexibility profiling, corresponding to that of larger flexible resources already employed in practice (e.g., distributed generators or storage), will make provision of multiple flexible services accessible to small and medium-size end-users. This will result in increased flexibility of the DN as a whole. Furthermore, harnessing flexibility potential of residential and commercial users would have significant environmental implications, as these contribute to a large share to both, electrical usage and global greenhouse gas emissions. The findings of the project could be further complemented with smart meter data to develop tariffs and incentives for residential and commercial users, supporting more coordinated procurement of flexibility by reducing uncertainty of efficiency and outcome of the demand response (DR) programmes. The main beneficiaries of the research would be DSOs, aggregators and other DR responsible parties at the DN level. The question of flexibility modelling is not only important for reporting DR potential at the demand side (commonly, an aggregator's role), but also for more confident estimation of the outcome of DR programmes, tariff design and flexibility assessment, which are highly relevant to DSOs. One of the main benefits for DSOs brought by this project would be in supporting decision making when investing into incentives and infrastructure allowing network-wide control of flexible loads.