Silicon microelectronics has been spectacularly successful at providing increasingly complex digital processing and large-capacity digital memory with continually improving performance/$. It has been less successful at providing simple function at very low cost and in providing electronic function over large areas or on arbitrary surfaces. Thin film transistors (TFTs), based on either inorganic or organic semiconductors, are of interest for lowcost flexible and arbitrary substrate applications. Inorganic amorphous silicon (a-Si:H) TFTs are widely used as pixel access devices in displays and large area sensors on glass substrates. It is also possible to fabricate these devices on polymeric or other unconventional substrates and we have demonstrated a-Si:H TFTs on high temperature polymeric substrates (for example, polyimide) with performance very similar to that obtained on glass substrates. Organic semiconductors are of particular interest because they can be deposited and processed at very low temperature (often less than 100°C). Organic thin film transistor (OTFT) device performance now rivals or exceeds that of a-Si:H devices, and low OTFT process temperatures allow fabrication on a range of surfaces including cloth, paper, or polymeric substrates. Using small molecule organic semiconductors we have fabricated TFTs with field effect mobility > 1 cm2/V-sec on flexible polymeric substrates with good uniformity and yield. Devices of either type (organic or inorganic) are of particular interest for applications where their switch characteristics can be used for selection or isolation of arrays of devices. This approach, widely used for display addressing, allows many thousands or even millions of sensor, actuator, or other elements to be controlled with simple, low-cost electronics and the flexible processing used for thin film active devices allows direct integration with a wide range of materials and devices.