The fabrication of complex metallic microstructures in 3D by injecting liquid solder into microfluidic channels and allowing the solder to cool and solidify is demonstrated; after fabrication, the metallic structures can be flexed, bent, or twisted (see figure and cover). With this method it is possible to build flexible electronic circuits, complex embedded or freestanding 3D metal microstructures, 3D electronic components, and hybrid electronic-microfluidic devices.