The purpose of this project is to determine the short-term effects of a customized Core Strengthening and Respiratory Exercise Program (CSREP) on children with cystic fibrosis (CF) between the ages of 10 and 21 who are receiving outpatient care at Children's Hospitals and Clinics of Minnesota. The CSREP, which will be provided by a physical therapist and a physical therapist assistant, consists of specific breathing techniques and core strengthening exercises, designed to improve rib cage mobility, pulmonary function, aerobic capacity, posture, and core strength. Currently the CF population at Children's receives physical therapy on an inpatient basis only.