The starting point of this thesis is the analysis - both through research and through design – of the ongoing and inevitable process of urban densification, in order to explore possible ways to face it with efficiency and quality. The goal is to investigate an alternative way to deal with the densification process in complex scenarios, where the traditional approach of simply substituting existing buildings for denser ones is not possible: historical city centers constitute local cultural heritage and for this reason need to be preserved, establishing huge constraints for this inner growth process. The central question raised is "how to accommodate the future city within the existing one", in a necessarily intimate relation between present and future buildings, improving the original quality of the space. In other words, the quest is how to promote an architectonic symbiosis able to provide positive outcomes to all the parties involved, or, in biological terms: a "Mutualitic relation". Exchanges of services and nutrients that define each kind of symbiosis in nature are translated into the architecttural field, defining relations of parasitism, amensalism, commensalism, neutralism, composite organism, evolution, mutual predation, competition and, finally, mutualism in architecture. A catalog explores 80 existing “symbiotic relations” between buildings in the current urban scene and between buildings and the city. Nowadays, incorrectly put together under the expression “parasitic architecture”, each example analysed was re-labeled according to proper analogies with the biological field. The outcomes of that analysis work as inputs for the “densification plan” for Amsterdam presented in a project complementary to this thesis, where two examples – under and above the heritage urban fabric – explore how to achieve a “Mutualistic Architecture”, with good outcomes to all parties involved.