Individuals with fragile X syndrome (FXS) demonstrate profound executive function deficits that interfere with learning, socialization and emotion regulation. Extensive research focused on the animal models of FXS show that targeted pharmacological agents can normalize synaptic connectivity and reverse cognitive and behavioral deficits. This translational work has led to multiple national and international controlled trials in humans with FXS now underway. However, in contrast to the heavy focus on medication treatments, there have been no controlled trials to empirically-validate cognitive or behavioral interventions for FXS. The proposed study, the first non-pharmacological controlled trial for FXS, will evaluate the efficacy of Cogmed, a cognitive training program proven to enhance working memory and executive/frontal function in a variety of clinical populations. Demonstration of effective Cogmed training for FXS would represent a major advance in the field, one that may also generalize to other forms of intellectual disability. Furthermore, it is critical to determine whether the targeted pharmacological treatments can accelerate learning and cognitive development. Thus, the validation of Cogmed for FXS will provide a paradigm for testing hypotheses focused on combined efficacy of medication and cognitive training.