<p>The 2D (two-dimensional) transition metal dichalcogenides (TMDs) are a novel category of materials with remarkable nonlinear optical (NLO) properties and hold transformative promise in the ultrafast photonic technologies. Their distinct band structures, high excitonic interactions, and excitonic direct-to-indirect bandgap transitions allow an efficient coupling of light and matter at atomic thicknesses. The nonlinear absorption coefficients, the third-order susceptibilities, the shortest possible response times, and optical limiting thresholds of ideal TMDs, such as MoS 2, WS 2, MoSe 2, WSe 2 and MoTe 2 were systematically studied. The findings show nonlinear responses that are material- and wavelength-dependent, with WSe 2 showing the strongest two-photon absorption and largest 3rd -order nonlinearity, meaning that it is extremely promising as an ultrafast modulator as well as telecommunication devices. WS 2 was a very good optical switch, and MoS 2 and MoSe 2 showed good saturable absorption characteristics, which are useful in mode-locked lasers. MoTe 2 had saturable and two-photon absorption, providing flexibility to all-optical processing. Their ability to perform high-speed optical operations is proven by the fact that their recovery times are below 250 femtoseconds in all samples. Moreover, strain, electric gating, and heterostructure engineering were demonstrated to be effective pathways to dynamically tune nonlinearities to maximize device integration flexibility. Experimental observations were enhanced by theoretical modeling, underscoring the significance of band structure engineering for optimal NLO performance. These findings establish TMDs as a versatile material platform for ultrafast photonics, bridging fundamental science with applied optical communications, laser technology, and nonlinear photonic devices.</p> Graphical abstract <p></p>