Electrical transport in ultrathin Cs layers on Si(001) has been studied combining macroscopic conductivity measurements with low-energy electron diffraction, energy loss spectroscopy, and measurements of the work function. At temperatures around 150K, growth of the first three atomic layers proceeds layer-by-layer. The completion of each layer correlates with stepwise increases of the surface sheet conductance with coverage. Calibrating the Cs coverage by combined conductivity and work function measurements, the areal density of a single atomic layer is determined as 0.5 monolayers (3.39×1014cm-2). Electron spectroscopy reveals a semiconductor-metal transition of the surface upon completion of the first atomic layer, which correlates with the onset of a macroscopically measured sheet conductance in the 10-5Ω-1 range. While the conductance can be ascribed to electrical transport within surface states, its dependence on temperature indicates an activation barrier, which, most likely, is due to domain boundaries. At coverages of one monolayer and beyond, the Cs Si(001) surface exhibits a high metal-like conductance in the 10-3Ω-1 range. © 2005 The American Physical Society.