Flame synthesis of nanoparticles provides access to a wide variety of metal oxide nanoparticles. Detailed understanding of the underlying fundamental processes is a prerequisite for the synthesis of specific materials with well-defined properties. Multiple steps from gas-phase chemistry, inception of first particles and particle growth are thus investigated in detail to provide the information required for setting up chemistry and particle dynamics models that allow simulating particle synthesis apparatus. Experiments are carried out in shock wave and flow reactors with in-situ optical diagnostics, such as absorption, laser-induced fluorescence, and laser-induced incandescence, with in-line sampling via mass spectrometry as well as with thermophoretic sampling for ex-situ microscopic analysis and electronic characterization. Focus is on tuning particle size as well as crystallinity and stoichiometry, with a specific focus on sub-stoichiometric materials with tunable composition.