We ana/lyze the performance of a recently reported Ge/Si core/shell nanowire transistor using a semiclassical, ballistic transport model and an sp³d⁵s* tight-binding treatment of the electronic structure. Comparison of the measured performance of the device with the effects of series resistance removed to the simulated result assuming ballistic transport shows that the experimental device operates between 60 and 85% of the ballistic limit. For this ∼15 nm diameter Ge nanowire, we also find that 14−18 modes are occupied at room temperature under ON-current conditions with I_ON/I_OFF = 100. To observe true one-dimensional transport in a ⟨110⟩ Ge nanowire transistor, the nanowire diameter would have to be less than about 5 nm. The methodology described here should prove useful for analyzing and comparing on a common basis nanowire transistors of various materials and structures.