We demonstrate that microfluidic flow devices enable a rapid, continuous, well-reproducible and size-controlled preparation of unilamellar block copolymer vesicles. The PDMS-based microfluidic device consists of perpendicularly crossed channels allowing hydrodynamic flow focusing of an ethanolic block copolymer solution in a stream of water. By altering the flow rate ratio in the water and ethanolic inlet channels, the vesicle size can be tuned over a large size range from 40 nm to 2 microm without subsequent processing steps manipulating size and shell characteristics. The ability of tuning the vesicle mean size over a range of several orders of magnitude with the possibility of in situ encapsulation of active ingredients creates new opportunities for the preparation of tailored drug delivery systems in science, medicine and industry.