Much of the world’s unexploited hydropower potential is in developing countries, many of whom are desperate for electricity and for water storage to support development. Yet these much needed projects are typically subject to extensive delays both before and during construction, which add to their cost and impacts and threaten their sustainability. Engineers have focused on the question of construction delay and made improvements in project management, but the implications of pre-construction delays are rarely documented or analysed. This analysis considers the impacts of pre-construction delay in hydropower projects and identifies how the adverse impacts of delay can be reduced to improve project sustainability. The impacts include obvious economic impacts on the countries concerned, but also less obvious environmental, social and even secondary economic impacts from project delays such as: the stress on communities of waiting years to be resettled; deforestation of project areas, lack of local investment and services and a disincentive for foreign investment caused by a lack of reliable power supply.