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 ribbons and wires of single crystal silicon, gallium arsenide, indium phosphide and gallium nitride. Multilayer three dimensional circuit configurations, heterogeneously integrated systems, GHz switching speeds and mechanical bendability as well as full stretchability represent a few of the unusual characteristics that can be achieved. These and other aspects, including the soft transfer printing techniques used to form the devices, will be discussed.