The overall aim of the thesis was to determine the prevalence and magnitude of relative age effect (RAE) in European female sport via examination of published data spanning 2011-2021. The PRISMA systematic search guidelines were used to identify 14 studies out of 68 containing 53 independent samples across 9 sport types. Based on frequency, percentages were moderated by age group, competitive level, sport type, sport context, and study quality. Additionally, this study analysed whether RAE was present or not by comparing relatively older (born in the first quarter of the year; Q1) with younger (born in the last quarter of the year; Q4) girls and women across all European female sport contexts. The thesis shows that the overall estimate identified a small-moderate RAE among European female sports, within subgroup revealed RAE magnitude was higher in pre-adolescent (≤ 11 years) and adolescent (12-14 years) age groups and at higher competition levels. The results show that RAEs in females’ sports are common across the European contexts. A major factor influencing RAE magnitude are developmental levels, competition levels, and sport context demands. To prevent RAE-related participation and attainment inequalities, it is recommended that sport policy, organizational structure, and athlete development system structures be changed as well as practitioner intervention.