In the hunt for ultimately thin electronic devices, atomically thin layers of group VI transition metal dichalcogenides (TMDCs) are recognized as ideal 2D materials after the success of graphene. Monolayer TMDCs feature nonzero but contrasting Berry curvatures and valence-band spin splitting with opposite sign at inequivalent K and K' valleys located at the corners of the 1st Brillouin zone. These features raise the possibility of manipulating electrons' valley and spin degrees of freedom by optical and electric means, which subsequently makes monolayer TMDCs promising candidates for spintronics and valleytronics applications.