Cerebral Palsy (CP) is a non-progressive condition; however, certain negative side effects such as a low muscle strength and cardio respiratory endurance can develop at later stages and can get progressively worse depending on the specifics of a person's condition. Children and adolescents with CP have distinctly subnormal aerobic and anaerobic capacity in comparison with typically developing peers. Also, muscle strength is reduced and energy cost of locomotion is high. Low levels on these fitness components may contribute to the difficulties in motor activities most children with CP encounter in daily life. Moreover, these changes can negatively influence the independence, skills and participation level of children and adolescents with CP. Newly developed fitness measures for children and adolescents with CP who are classified at Gross Motor Function Classification System (GMFCS) level I and II are being examined on its validity and reliability. All tests were based upon the specificity of testing principle. This means that the modality of the testing tool needs to be similar to the type of activity the subjects train or perform in. This means, since the tests were developed for children who are able to walk independently (GMFCS level I or II), that the aerobically and anaerobically based tests were running-based, and that the lower extremity strength tests were centred on functional exercises that can be performed by the same subjects. The 10-m Shuttle Run Tests (SRT), the Muscle Power Sprint Test (MPST) and the 10x5 Meter Sprint Test are valid and reliable running-based tests to measure respectively the aerobic, anaerobic capacity and agility. The 30-sec Repetition Maximum (30-sec RM) showed acceptable reliability when used for measuring the lower extremity muscle strength. A randomized controlled trial (RCT) was performed to examine the effects of an eight-months training program with standardized exercises on aerobic and anaerobic capacity in children and adolescents with CP. A total of 86 children with CP (aged 7-18 years) classified at GMFCS-level I or II participated in this study. Thirty-two of the children underwent a standardized training program of aerobic and anaerobic exercises for 8 months, whereas 33 children in the control group received standard rehabilitation treatment. Those in the training group showed significant improvements in aerobic and anaerobic capacity, agility, muscle strength, and athletic competence. In a quality-of-life assessment, the children in the training group also showed significant improvements in basic motor functioning, autonomy, and cognitive functioning. The intensity of participation showed a similar effect for the formal, overall, physical and skilled-based activities. Hence, an exercise training program improves physical fitness, the participation level and health-related quality of life in children with CP when added to standard care. We concluded that an eight-month standardized exercise program consisting of functionally based exercises significantly improves physical fitness, the intensity of activities and health-related quality of life in children with CP when added to standard care.