Studies on the diagnosis of pathologies of concrete dams are a valuable reference for professionals managing such assets. However, detailed information is rarely reported in the literature. This paper compiles the most important lessons learned during 30 years of practical experience on this area. The diagnostic procedure is illustrated through 4 of the most interesting dams analysed and monitored by the authors. All were subjected to a first stage that included inspection and analysis of historical documentation, leading to the proposal of a diagnose hypothesis. This was followed by a second stage that included an experimental programme with laboratory tests and, in some cases, numerical simulations to confirm or reject the hypothesis. The analysis of the information shows different pathologies and provide a wide spectrum of boundary conditions, symptoms and diagnoses (from internal sulphate attack and alkali-aggregate reaction to soil-structure interaction).