This final work describes hydroelectric power plants, what they serve, how they work, what they help and what they retaliate against, how are they build and what function they have in people's lives, but also in other living beings. Also, in this final work was elaborated all parts of hydro power plants and their functions in the operation of the complete plant as well as the division of hydro power plants according to various factors like modes of operation, positions, and similar. The final work describes the complete operation of the hydropower plant from water accumulation to water acceptance, water supply, water passage through turbines and engine room, electricity production through generators and its distribution through transformers and water drainage into further natural flow. At the end of the final work, the hydroelectric power plant Jaruga is described in detail, officially the oldest hydroelectric power plant in Europe and the second in the world, all its parts and the way of work as well as the repair of its external, visible and some, long-hidden parts, supported by pictures before and after the repair. Most of the reparation is related to the aesthetic nature because the hydropower plant itself is located at the foot of the most beautiful set of waterfalls of the Krka River and is often found on national park postcards while the rest of the reparation refers to improved operation of the hydropower plant.