This is a Phase 3, randomized, double-blind, placebo-controlled study to evaluate the efficacy, safety, and tolerability of crinecerfont versus placebo administered twice daily (bid) with breakfast and evening meals for28 weeks in approximately 81 pediatric subjects with classic congenital adrenal hyperplasia (CAH) due to21-hydroxylase deficiency. Eligible subjects will be randomly assigned in a 2:1 ratio (active:placebo) to either crinecerfont (25 mg bid via oral solution for subjects 10 to <20 kg, 50 mg bid via oral solution for subjects 20 to <55 kg, or 100 mg bid via oral capsules for subjects ≥55 kg) or matching placebo (oral solution placebo for subjects <55 kg and oral capsule placebo for subjects ≥55 kg). Dose assignment from Day 1 to Week 28 will be based on the subject’s weight at Day 1. After the 28-week placebo-controlled treatment period, there will be a 24-week, open-label treatment period, during which all subjects will receive crinecerfont at doses based on their Week 28 body weight.