Very fast mixing in the range of milliseconds as well as deliberately slow mixing was realized by specially adjusted interdigital micro mixers made of glass or stainless steel. The corresponding micro mixers are presented including experimental and theoretical investigations of the respective mixing process. Fast mixing was realized by combination of flow multilamination by interdigital microstructured feeding structures with geometric focusing. Details on the microfabrication, achievable throughputs and hydrodynamics are discussed. To prevent clogging of microsized feeding structures in the case of precipitation reactions, mixing was deliberately slowed down by separating the reactant solutions at the outlet by additional layers of inert liquids.