Device physics in organic thin-film transistors (OTFTs) have been extensively studied along with the efforts to improve device performance and to develop their applications. Among many promising organic semiconducting materials, pentacene has been a benchmark in this re-search field. Understanding the bottlenecks of carrier transport in pentacene OTFTs is therefore important to maximize the performance of OTFTs. In this paper, we summarize our knowledge on the bottlenecks against carrier transport in practical polycrystalline organic thin films: ex-trinsic factors that disturb the carrier transport in OTFTs, intrinsic structure and properties, equations to express the overall carrier mobility in polycrystalline films, and influence of sur-face chemical modification on the crystallographic and electronic structures.