Two-dimensional (2D) semiconductors have attracted considerable attentions from electronic engineering community due to their unique electronic properties. Especially, the inherent advantage of scaling semiconductor into atomic thickness has raised the prospect of possible extension of the Moore's law. To achieve 2D electronics, a full comprehension of semiconductor defect physics and chemistry is indispensable due to its decisive role in controlling electrical performance of 2D materials and functionalizing their devices. In this review, first we explain why 2D semiconductors is important for nanoelectronics and optoelectronics. Second, we elucidate how native defects or intentional impurities affect and control electrical characteristic in 2D semiconductors, such as carrier concentration and their conductive type. In this section, experimental pictures of defects and several updated theoretical methods to evaluate ionization energies of defects and their conductive type are introduced in detail. Third, typical device experiments are shown to demonstrate a direct role of defects to functionalize 2D electronic device. Furthermore, a database of popular defects and their electrical properties in current popular 2D semiconductors is summarized for references. Last, we discuss the challenges and potential prospects of defect engineering for 2D devices. The present paper offers important viewpoints from semiconductor defects to design the emerging 2D electronics. (C) 2017 Elsevier Ltd. All rights reserved.