PEP stands for Positive Expiratory Pressure. Bubble PEP is a treatment to help children who have a build up of mucus in their lungs. Bubble PEP is used for any child who has difficulty clearing mucus, most commonly those with cystic fibrosis (CF) or those who have just had surgery. During treatment, your child will be encouraged to blow big bubbles through water – which is fun for them! Treatment is explained in the next section; and the equipment used is shown in the diagram below.