Moiré superlattices can be used to engineer strongly correlated phenomena in van der Waals heterostructures, as recently demonstrated in graphene-based systems. Aligned transition metal dichalcogenide (TMDC) heterostructures may also support correlated electronic states, with additional opportunities arising from the strong-light matter interaction and large spin-orbital coupling. In this talk, I will discuss our observation of correlated phases in semiconducting WSe2/WS2 moiré superlattices. Using a novel optical detection technique, we reveal a Mott insulator state at one hole per superlattice site and surprising insulating phases at fractional filling, which we assign to generalized Wigner crystallization on the superlattice. Further, we demonstrate that the spin-valley optical selection rules in TMDCs can be used to optically create and study low-energy spin excitations in the correlated states.