Since the discovery of 2D materials, two-dimensional transition metal dichalcogenides (TMDs) have attracted significant attention due to their rich exciton physics and optically addressable valley/spin degree of freedom of carriers. This dissertation presents our comprehensive optical spectroscopic study on excitons and valley-dependent phenomena in monolayer and twisted bilayer TMDs. The first part of the dissertation focuses on the valley polarization of excitons, exciton polarons, and carriers in monolayer TMDs. The valley dynamics are explored through electrical gating, temperature tuning, magnetic field application, and optical pumping. We demonstrate the tunable valley polarization of excitons/carriers and discuss the relevant mechanisms, highlighting their promising potential in optoelectronics and valleytronics. The second part delves into twisted bilayer TMD moiré superlattices, specifically investigating the spin and valley properties of moiré-trapped interlayer excitons in the WSe2/WS2 moiré superlattices. We identify these excitons' spin/valley configurations and probe their multiparticle nature. The work lays the groundwork for exploring interactions among excitons, phonons, and charge carriers within the quantum-correlated phases hosted by the moiré superlattice.