Doping is essential to manipulate the electrical performance of both thermoelectric (TE) materials and organic semiconductors (OSCs). Although organic thermoelectric (OTE) materials have experienced a rapid development over the past decade, the chemical doping of OSCs for TE applications lags behind, which has limited further breakthroughs in this cutting-edge field. Recently, increasing efforts have been devoted to the development of energetically matched host and dopant molecules, exploring novel doping methods and revealing the doping mechanisms. This tutorial review covers the basic mechanisms, fundamental requirements, recent advances and remaining challenges of chemical doping in OSCs for TE applications. We first present the basic knowledge of the trade-off relationship in TE materials and its critical requirements for doped OSCs, followed by a brief introduction of recent advances in the molecular design of OSCs and dopants. Moreover, we provide an overview of the existing categories of doping mechanisms and methods, and more importantly, emphasize the summarized doping strategies for the state-of-the-art OTE materials. Finally, challenges and perspectives on the chemical doping of OSCs are proposed to highlight the research directions that deserve attention towards a bright future of OTE materials.