ABSTRACT: This paper describes the heightening project of a 114 m high arch-gravity dam by 23 m located in central Switzerland. The heightening of the dam is part of a larger project aimed at optimising the use of hydropower at low cost and minimum risk to investors. The existing structure presents behavioural anomalies which are related to the design and the ageing of the structure and therefore the heightening of the dam also englobes its rehabilitation leading to a dam design which fulfils both future water-retaining objectives and long-term safety requirements. Following an overview of the existing structure, the main project stages leading to the design of the heightened dam are presented. Particular attention is given to the overview project configuration, preliminary dam shape optimisation and monitoring of the structure.