Two-dimensional layered transition metal dichalcogenides (TMDCs) have demonstrated a huge potential in the broad fields of optoelectronic devices, logic electronics, electronic integration, as well as neural networks. To take full advantage of TMDC characteristics and efficiently design the device structures, one of the most key processes is to control their p-/n-type modulation. In this review, we summarize the p-/n-type modulation of TMDCs based on diverse strategies consisting of intrinsic defect tailoring, substitutional doping, surface charge transfer, chemical intercalation, electrostatic modulation, and dielectric interface engineering. The modulation mechanisms and comparisons of these strategies are analyzed together with a discussion of their corresponding device applications in electronics and optoelectronics. Finally, challenges and outlooks for p-/n-type modulation of TMDCs are presented to provide references for future studies.