Pentacene is one of the most extensively studied semiconducting materials for organic thin-film transistors (OTFTs) and has been still a benchmark material in this research field. Although precise studies on the structure and electronic properties of pentacene single crystals give us exact knowledge about this material, there exists a large gap between the fundamental physics of single crystal and device characteristics brought by practical polycrystalline films. Understanding the bottlenecks of carrier transport in pentacene OTFT is therefore important to maximize the performance of OTFTs utilizing not only this benchmark material but also any semiconducting small molecules. In this chapter, the reality of the carrier transport band in practical polycrystalline organic thin films is explained by making pentacene into a representative case, mainly based on our efforts of ten-years. The major topics included are as follows: grain morphology and crystal structure of pentacene thin films, extrinsic factors that are more-or-less introduced and disturb the carrier transport in OTFTs, intrinsic structure and properties of pentacene polycrystalline thin films, equations to express the overall carrier mobility in polycrystalline films, and influence of surface chemical modification on the crystallographic and electronic structures.