This chapter examines the clinical pharmacology study design and need for pharmacokinetic (PK) assessment for topical products in pediatric subjects. The chapter focuses on several design elements that create the building blocks of the maximal usage trial (MUsT), and some of these individual components are supported by examples to enhance the understanding of these concepts. The MUsT study is the primary clinical pharmacology study for topical dermatological products. Several examples discussed in this chapter do suggest that there is significant value in pediatric MUsT studies related to support for drug approval, safety, and dosing instructions in terms of limitations of % BSA and amount applied. Examples provided include sunscreens, topical corticosteroids, retinoids, and pediculicides. The goal of safe and efficacious products in pediatric dermatology can be achieved through the performance of controlled clinical trials in the pediatric patient population.