Objective: This presentation describes a set of studies examining sex differences in brain development in the domains of language and behavioral control. Neuroimaging and cognitive test scores were collected from 452 children (3mos to 18 yrs; 219 male) as a part of the NIH MRI study of normal development. Children were administered language tests that included DAS naming vocabulary, the Woodcock-Johnson, the California Verbal Learning Test (CVLT) and the Wechsler scales. Behavioral ratings included the Behavior Rating Inventory of Executive Function (BRIEF) and the Child Behavior Checklist (CBCL). The neuroimaging studies included gray and white matter volumetric studies and Fractional Anisotropy (FA) and Mean Diffusivity (MD) derived from DTI. Analyses included curvilinear regression of structural imaging and cognitive tests utilizing 115 ROI's from the AAL template and 48 ROI's from the Johns Hopkins White Matter Atlas. Separate developmental curves were constructed for males and females of all variables under study. There were significant differences in male and female development using gray and white matter densities with males having higher values across development, reflecting larger brain size. Development curves derived from DTI analyses indicated that males and females have a similar pattern of brain maturation. However, they differed on a number of brain areas, include those subsuming language in the left hemisphere and executive functions in the frontal lobes. In particular, the development of the left hemisphere and language function was associated with parent ratings of greater behavior control. These studies have a bearing on the design of cognitive tests and the general interpretation of test scores.