We present low-temperature magneto-photoluminescence experiments which demonstrate the brightening of dark excitons by an in-plane magnetic field B applied to monolayers of different semiconducting transition metal dichalcogenides. For WSe2 and WS2 monolayers, the dark exciton emission is observed at ∼50 meV below the bright exciton peak and displays a characteristic doublet structure whose intensity grows with B^2 , while no magnetic field induced emission peaks appear for MoSe2 monolayer. Our experiments also show that the MoS2 monolayer has a dark exciton ground state with a dark-bright exciton splitting energy of ∼100 meV.
