Portable transdermal electrical stimulation (TES) applicators for modifying a subject's cognitive state. In general, the portable applicators described are specifically configured and adapted to be lightweight and may be wearable, and to deliver a high-intensity TES able to evoke or enhance a predetermined cognitive effect. These TES applicators may include a pair of electrodes and a TES control module comprising a processor, a timer and a waveform generator. TES control module is adapted to deliver a biphasic electrical stimulation signal of 10 seconds or longer between the first and second electrodes having a frequency of 400 Hz or greater, a duty cycle of greater than 10 percent, an intensity of 3 mA or greater, with a DC offset.