Semiconductor nanowires have attracted considerable attention during the last decade and are considered as an alternative path to extend the road for scaled semiconductor devices. The interest is motivated by the improved electrostatic control in the cylindrical geometry and the possibility to utilize heterostructures in transistor design. Currently, nanowire transistors have been realized both in III-Vs and in group IV materials employing top-down as well as bottom-up technologies. In this review, we give an overview of the field and, in particular, we summarize state-of-the-art for III-V nanowire devices. It is demonstrated that the growth and processing technologies are maturing and that devices with good transistor characteristics are being fabricated by a combined bottom-up and top-down approach. Also, the first radio-frequency (RF)-implementations are reviewed and discussed.