The synthetic engineering of biofilms can lead to processes with increased productivities, biofilm matrices with better properties or the tailor-made interaction of microorganisms. Meanwhile, we have molecular tools to engineer the synthetic interaction of organisms by designing the share of labour or by precipitating organisms with each other. Moreover, bioprinting gives us the possibility to print organisms in defined mixtures and densities and with specifically adapted inks. Nevertheless, the question on the long-term applicability of synthetic biofilms or organisms in synthetic matrices has not been conclusively answered yet. At biofilms 9, we hope to discuss results and maybe answer fundamental questions with respect to the synthetic development of biofilms like how can we achieve long-term interactions in biofilms, what defines an advanced synthetic biofilm matrix, or what are the benefits that we can gain by the synthetic development of tailored biofilm communities and structures.