Traditionally, the transmission network transported electricity over extended distances from local large scale electricity generation plants to distribution networks that transported electricity to end consumers (Ekanayake, 2012). In order to ensure that such a system remains operational, a Distribution System Operator (DSO) maintains, operates and invests in the grid at the distributional level. One of the reasons grid investment is performed is to prevent grid congestion that occurs when the electricity load exceeds the grid capacity. Investing in reinforcing the distribution grid components (cables, transformers, fuses, etc.) increases the capacity of the electricity grid, which, in turn, prevents grid congestion. However, with the advent of renewable energy sources, decentralized generation units and electrical appliances (electric vehicles, heat pumps, electric boilers etc.), there is an increasing pressure on modern electricity networks because of overproduction at the local level that leads to reverse transfer of power, increase in grid losses, and voltage and current fluctuations. This increasing pressure on the electricity grid could be reduced by grid reinforcement; however, it is argued that grid reinforcement and investments cannot keep up with the growth of intermittent renewable energy sources, which may result in interim and short term congestion. Moreover, Haque et al. (2014) claimed that upgrading grid assets, which is considered capital intensive, will not serve as a cost-effective solution in modern grids, as the electricity network congestions are temporary. Although the integration of renewable and distributed resources increases the complexity of operation and preservation of the reliability of the grid, it also provides opportunities to manage the load on the network. These opportunities surface and evolve from the flexibility that results from Demand Response (DR), also referred to as Demand-Side Flexibility. Flexibility, provided by DR, is created by controlling the distributed energy resources and electrical appliances (heat pumps, electric boilers etc.) on the distribution network, which may potentially reduce peak loads or shift loads to off peak periods of time. With flexibility from such decentralized electricity sources and appliances, it is possible to manage the electricity load variability in a more cost effective manner, which may result in postponing capital intensive grid reinforcement (Ecofys, 2015). A third party, called the aggregator, is responsible to aggregate the flexibility from controlled devices and sources. Therefore, the DSO engages in short/long term contracts with the aggregator to procure the flexibility to resolve congestion. Although demand-side flexibility holds potential in preserving network reliability by mitigating congestion, and thus postponing grid investment, the impact of demand-side flexibility provided by Demand Response on congestion mitigation, from a technical perspective, is blurred and indeterminate (Moslehi & Kumar, 2010). Moreover, the prospects of financial savings for the DSO are not guaranteed (Torriti et al., 2010). Therefore, to bridge this knowledge gap, the thesis explored the following main research question: “To what extent can the DSO mitigate grid congestion by means of Demand-Side Flexibility to defer grid reinforcement?”