This chapter focuses on the Toolbox Pattern Comparison Processing Speed Test, which was developed to assess processing speed within the NIH Toolbox Cognition Battery (CB). We describe the development of the test, highlighting its utility in children. In addition, we examine descriptive data, test–retest reliability, validity, and preliminary work creating a composite index of processing speed. Results indicated that most children were able to understand the basic concepts of the Toolbox Pattern Comparison Processing Speed Test. Further, test–retest reliability was excellent. Analyses examining convergent and discriminant validity provided support for the utility of the test as a measure of processing speed. Finally, analyses comparing and combining scores on the Toolbox Pattern Comparison Processing Speed Test with other measures of simple reaction time from the NIH Toolbox CB indicated that a Processing Speed Composite score performed better than any test examined in isolation. Taken together, the Toolbox Pattern Comparison Processing Speed Test appears to exhibit a number of strengths: it is child-friendly and engaging, short and easy to administer, and has good construct validity, especially when used as part of a composite score.