The use of models in different stages of a process life-cycle is described. These stages are partly overlapping and have feedback between them. The scope may range from calculation of mass and energy balances of a single unit operation to large flow sheet simulation and optimization. The novelty of the case at hand or an innovative approach taken may require the development of tailor-made, detailed new models. Sub-problems include choices between competitive theories, mechanistic and empirical approach and the proper degree of details. The same models should be applicable at different stages and for different tasks, such as simulation, optimization, design, experiment planning etc. Development of the melamine and hydrogen peroxide production processes are used as examples.