The present invention relates to microfluidic devices that comprise a 3-D microfluidic network of microchannels of arbitrary complexity and to a method for fabricating such devices. In particular, the invention relates to a method of forming microfluidic devices having 3-D microfluidic networks that contain open or closed loop microchannels using a single-step molding process without the need for layer-by-layer fabrication, and to the resultant microfluidic devices. The networks of such microfluidic devices may comprise one or more microchannel circuits which may be discrete or interconnected.