This chapter focuses on the NIH Toolbox List Sorting Working Memory Test, which was developed to assess processing speed within the NIH Toolbox Cognition Battery (CB). This test is a sequencing task requiring children and adults to process stimuli (presented both visually and auditorily) and sequence the stimuli according to size. We describe the development of the NIH Toolbox List Sorting Working Memory Test, highlighting its utility in children. We examine descriptive data, test–retest reliability, and convergent and discriminant validity. Results indicated that List Sorting performance was positively correlated with age indicating that performance on the task improved throughout childhood and early adolescence. Further, test–retest reliability coefficients were high and there was support for both convergent and discriminant validity. These data suggest that the NIH Toolbox List Sorting Working Memory Test is reliable and shows evidence of construct validity.