One of the most promising applications of organic field-effect transistors (OFETs) is chemical and biological sensing. In order to achieve highly sensitive and selective detection of a broad array of analytes, the organic semiconductor layer in an OFET is tailored with rationally designed physical structures and chemical functionalities. This article reviews representative studies on structured and functionalized organic semiconductors after a brief introduction to OFET-based chemical and biological sensors. Finally, we reach conclusions on the achievements and challenges in this research field and provide an outlook for the development in the near future.