This essay is about hydropower and dam constructions, which are important subjects nowadays. Dam constructions increase in number around the world and it is important to learn about their effects and possibilities. This essay contains general information about hydropower and the different effects from dam constructions regarding social, environmental and economical aspects. The purpose is to investigate how the dams effect people and the environment and to induce future aspects about the subject. A comparison has been made between two types of countries, industrialised and developing countries and two case studies have been done to get information about the different situation. Dam constructions are built over the world and there are several advantages and disadvantages with them regarding the three areas social, environmental and economical. Examples of advantages with hydropower are that it is a renewable source of energy and that it is relatively easy to regulate the water flow depending on the demand of power. Example of a disadvantage with hydropower is that the flow of water fluctuates and that brings about an uncertainty in the power production during the year. Positive effects from the dam constructions are that the water stored in the reservoirs could be used for irrigation and for power production. Negative effects are that the dams have negative consequences on the environment, such as erosion, flooding and low values of nutrient downstream the dam. Another important negative aspect is that people living along the rivers have to give up their land.