The valorisation of biomass residues is a challenge andValorization an opportunity. The large number of possible added value productsAdded value products and electricity that can be obtained results in a difficult product and process designProcess design problem. Integrated processes would allow fully reutilization of the residues, providing, at least, part of the utilities and intermediates are internally produced. Systematic process synthesisProcess synthesis techniques are presented to show the development of integrated processes towards circularity. In addition, with residues, building a circular economyCircular economy occurs at different levels, at process level, reusing the waste to produce the utilities and to recover nutrients that would help produce the next harvest, and along the foodFood chain, so that the different echelons of the chain are integrated towards developing a circular economyCircular economy. This chapter presents the mathematical optimization approach developed for the simultaneous process and product design and several examples are presented to show the application to the valorisation of biomass such as orange peels, wine, coffee and oil production residues, or synthesizing products from manure developing fully integrated processes. In addition, a larger scale has also been considered such as the integration of the operation of farms and agricultural exploitation defining the animal feed for the optimal integration of the different business and reducing the impact by reducing the need for inorganic fertilizers.