Atomically thin layers of materials, which are just a few atoms in thickness, present an attractive option for future electronic devices. Herein we characterize, optically and electronically, atomically thin tungsten disulphide (WS2), a layered semiconductor. We provide the distinctive Raman and photoluminescence signatures for single layers, and prepare field-effect transistors where atomically thin WS2 serves as the conductive channel. The transistors present mobilities μ = 10 cm2 V−1 s−1 and exhibit ON/OFF ratios exceeding 100000. Our results show that WS2 is an attractive option for applications in electronic and optoelectronic devices and pave the way for further studies in this two-dimensional material.