Solution processable conductors, dielectrics and semiconductors represent enabling materials for electronic circuits that can be fabricated on plastic sheets by continuous, high speed printing techniques. It is generally believed that these types of systems, which can cover large areas, will be important for new applications in consumer electronics. This talk describes the operational aspects of flexible transistors and circuits that use printable semiconductors based on silicon, gallium arsenide, indium phosphide, carbon nanotubes and organic polymers. It summarizes the soft lithographic techniques used to fabricate these systems, and demonstrates their use for patterning with resolution at the scale of an individual macromolecule (1-2 nm).