eROSITA is the main instrument on-board the Russian/German "Spectrum-Roentgen-Gamma" (SRG) mission. It will perform the first imaging all-sky survey in the medium energy band. The development of eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is a national German project, funded by MPG (Max-Planck Gesellschaft), co-funded by DLR (Deutsches Zentrum für Luft- und Raumfahrt) and implemented by MPE (Max-Planck Institut für extraterrestrische Physik) in cooperation with other German institutes and industry. Different from other such extensive and challenging missions, MPE took the responsibility for the overall management, system engineering and product assurance and sub-contracted only specific development tasks, manufacturing and testing for the benefit of small companies. MPE is the architect and the user of this high tech instrument. MPE defined the requirements and verified the requirements. This is a non-standard situation but offers much more flexibility in interpretation of requirements and tailoring of solutions. The implementation of the project was driven by lean management with close cooperation of technical and scientific personnel with pragmatic approaches and extensive testing, supported by the magnitude of inhouse facilities and tools. Potential solutions were discussed, evaluated and selected by mutual agreement. Nonconformances were treated in the same pragmatic way. The paper reports on how eROSITA was made, from design to the realization of the flight model and how it was delivered to and integrated in SRG and how it was tested at the prime contractor. The paper will describe how MPE as a scientific institute managed this challenging project including project management, system engineering and product assurance. The paper will also report on the very positive cooperation but complex communication and negotiations with the prime contractor in Russia.