In the past 10 years, a growing body of evidence has linked fundamental motor skills (FMS) proficiency to physical activity participation. With FMS proficiency becoming more significant for understanding factors that may influence young people's physical activity, instruments and testing methods used in such research must be valid and reliable. This study provides valuable information on interrater assessment in the field for a process-oriented FMS test battery on adolescents. It also highlights some of the barriers in determining reliability values in live observation field-based research rather than in more controlled research environments. Future studies assessing the FMS proficiency of children and adolescents should aim to assess interrater objectivity overall and for each skill tested.