Traumatic brain injury (TBI) and post-traumatic stress disorder (PTSD) are common combat-related conditions that can result in impairments in cognition, including memory, attention and multi-tasking. There are few effective interventions to improve cognition in Veterans with cognitive impairment due to these conditions. Non-invasive brain stimulation techniques, like transcranial direct current stimulation (tDCS), can be used to stimulate activity in areas of the brain to optimize recovery and rehabilitation. In this study, the investigators are conducting a randomized controlled trial of tDCS to improve cognitive function in Veterans with cognitive impairment due to co-morbid TBI and PTSD. TDCS is remotely administered and paired with cognitive training.