The Feature Article by Sirringhaus et al. (pp. 1655–1676) is focused on understanding the charge-transport physics of high-mobility organic semiconductors at a molecular level. Recent high-mobility small-molecule and conjugated polymer materials are reviewed, with a focus on crystalline materials that have been able to exceed mobilities of 0.5-1 cm2/V s. The authors discuss some of the main, competing factors that govern charge transport in these materials and present theoretical approaches that have been developed to describe systems in which moderately strong intermolecular electronic interactions and strong electron-phonon interactions are present. Finally, recent experimental results are reviewed that address the important question of whether at room temperature the charge carriers in these high-mobility organic semiconductors are in fact simply extended Bloch electrons that undergo occasional scattering processes, or are localized on individual molecules and move by hopping.