Conventional electronics relies on intrinsic properties of electrons,i.e.either charge or spin degrees of freedom.However,breakthroughs in future electronics are likely to be levered with new paradigms of operation by exploiting degrees of freedom other than charge and spin.The valleys,degenerate energy extrema of Bloch bands in momentum space,correspond to a well-defined degree of freedom for low energy electrons in many crystals.Drawing close analogy to already extensively studied spintronics,the parallel concept ofvalleytronics naturally has immense potential,aiming to utilize the valley index of carriers to encode information.For halfa decade after the inception of this concept,there have lacked approaches for dynamical control of the valley degree of freedom,which is necessary for processing information.However,the past year has seen ground breaking theoretical and experimental breakthroughs on dynamical control of valley in a new class of 2D crystalline semiconductors,i.e.atomically thin layers of transition metal chalcogenides.