This paper discusses techniques, limitations and possible future developments of circuits based on transistors operated in the weak inversion (w.i.) mode, also called subthreshold mode. In analog circuits, w.i. is reached at very low current, but it is also needed for very low supply voltage. Its exponential behaviour can be exploited in special circuits schemes, some of them devised for bipolar transistors. For digital circuits, it can provide the ultimate speed/power ratio in a given process.