Study Design/Procedure: The proposed study uses a double blind, parallel, placebo control design to test the hypotheses that augmentation of suppressed NMDA receptor function following pediatric msTBI with D-cycloserine (DCS) will increase neural activation, and when used in conjunction with cognitive training will enhance the effect of cognitive training on working memory (WM). Participants who show WM weaknesses within 12 to 24 months post-injury will be randomly assigned to one of two arms in the proposed study. Weakness in WM is operationally defined as a working memory index (WMI) standard score below 90 or evidence of at least a 10-point discrepancy between IQ and WMI. Given the purpose of the study (improving working memory), there is little utility in including children who do no demonstrate weaknesses in WM. Participants in both treatment arms will complete baseline evaluations on the WM and executive functioning (EF) tests that assess generalization, and parents will complete questionnaires that assess WM and attentional functioning. (Note: participants who completed their 12-month RAPBI visit within three months of enrollment in the current project will complete only novel measures). All participants will then receive their first Cogmed training session. During this first visit, participants will be also complete our MRI protocol immediately before and two hours after the first dose of DCS or placebo to measure the acute physiological response to DCS. Both groups will then complete six weeks of Cogmed training and in addition either DCS or placebo. Cogmed training will be completed in the home, with the parent/guardian supervising completion. During the six weeks of training, both you and your child will speak with a coach once a week, who will guide your child through the program and help to monitor his/her progress. Participants will travel to UCLA half-way through their training to consult with one of the medical personnel regarding the medication and to note any side effects. At the end of the sixth week, participants will be retested on our MRI protocol and non-trained measures of WM and EF. Parents will complete the questionnaires that assess WM and attention. Three months after the termination of treatment, participants will again return to UCLA to complete one session of Cogmed training and will be retested on the non-trained measures of WM and EF. Their parents will complete questionnaires to assess the maintenance of training.