Microreactor systems enable various process steps to be combined in the smallest of spaces. This chapter presents the individual functional units of such microfluidic systems and their manufacturing processes – in particular, the modern, high-resolution 3D printing technique. In addition, possibilities and restrictions of liquid transport, mixing or separation, the establishment of defined residence time and reaction paths, as well as the integration of sensors in lab-on-a-chip systems are demonstrated. Finally, examples from current research show how microfluidic systems can be used for analytical questions or in bioprocess technology.