Atomically thin 2D crystals has recently emerged as a new class of nanomaterials with fascinating optoelectronic properties. In particular, the valley state of electron in 2D transition metal dichalcogenides (TMD) has been considered as a novel information carrier in addition to spin and charge. The valley electronic states are shown to be easily accessible via circular polarization of photoexcitation. In addition, the intrinsic valley lifetime is expected to be remarkably long due to the unique spin-valley locking behavior. Here, I will present our recent studies to understand and control valley states by utilizing ultrafast optical spectroscopy. First, we demonstrate that non-resonant femtosecond optical pulse can induce ultrafast and strong effective magnetic field which can provide novel control of valley states. Effective magnetic field induced by femtosecond pulse can be more than 60 T. Second, we report efficient generation of microsecond-long lived valley polarization via ultrafast charge transfer process in WSe2/MoS2 heterostructures. This valley polarization has near unity degree and several orders of magnitude longer lifetime than previously reported values. Our ultrafast optical studies reveal promising potential of valley electronic states in TMD monolayers for novel valleytronics and spintronics applications.