Insect repellent should be applied under sunscreen. Sunscreen should be applied first to protect against harmful UV rays, and then insect repellent can be applied underneath. By applying sunscreen first, you ensure that your skin is adequately protected from the sun's rays. Applying insect repellent on top of sunscreen may reduce its effectiveness. Remember to reapply sunscreen and insect repellent as directed on the product labels throughout the day for optimal protection.