It is fairly well established that fly ash can improve many of the properties of fresh and hardened concrete as well as reduce the greenhouse gas (GFG) footprint associated with the use of Portland cement. However, the use of fly ash at higher replacement levels may produce some undesirable properties such as a slower set time and strength development or increased carbonation or salt scaling. Consequently there is a need to optimize the fly ash content of concrete for different applications. This paper discusses how the optimum the level of fly ash is dependent on the properties of the fly ash, the performance requirements for the fresh and hardened concrete, the climatic conditions during construction and the exposure conditions and durability requirements during service. In many cases, the optimum level of fly ash may be 40% or more.