During the design of ammonia and urea plants many aspects have to be taken into account besides process performances. The plant reliability is a factor as important as process performances in determining the future plant profitability. To design a reliable plant it is necessary to take into account many aspects, such as the process mechanical design of all parts, instrumentation, process control, and the plant layout. It is also essential to have a good quality control in every phase of the project. This paper describes the design choices that are crucial in order to reach the highest standard of reliability in these plants.