Underarm throwing is one of the most common throwing patterns used in many sport activities (e.g., bowling ball delivery, field hockey drives and passes, horseshoe pitch, underarm volleyball serve, badminton underarm clear and windmill softball pitching). However, there is very limited research on underarm throwing and even less research on the age- and experience- associated differences in children's underarm throwing. Current studies in underarm throwing have focused on the injury aspect of this motion and a majority of the subjects have included athletes at the collegiate level. Questions remain relative to how age and practice influence the development of underarm throwing and windmill pitching. A better understanding of the developmental characteristics of these skills may facilitate instruction and practice techniques. Therefore, the goal of this study is to qualitatively and quantitatively analyze age- and skill- related characteristics (forms and techniques) of young females' underarm throwing and windmill softball pitching. A total of 48 young females participated in this research: 25 untrained (UT) participants (M=9.69 years of age), 14 trained/league (TL) participants (M=10.91 years of age, training experience about 1.32 years), and 9 trained/formal (TF) participants (M=10.44 years of age, training experience about 1.03 years). In addition, three female collegiate pitchers (M=18.7 years of age) voluntarily took part in this study, serving as matured performers in the windmill pitching skill. Movement characteristics of arm motion, trunk activity, wrist snap and foot action were analyzed to capture experience and age- related differences of these skills. Qualitative analyses revealed that the formally trained participants demonstrated more mature performances when compared to their untrained and league trained counterparts. Quantitative analyses, utilizing the technique of analysis of variance (ANOVA), revealed training/experience to be a significant factor among the participants relative to trunk rotation, F (2,47)=53.94, p < .0001. The age and training/experience of the participants resulted in significant differences between the groups for the foot action component, F (2,47)=7.24, p < .002; F (2,47)=59.93, p < .0001, respectively, and for the arm component, F (2,47)=7.46, p < .002; F (2,47)=224.32, p < .0001, respectively. Finally, quantitative analyses revealed that only training/experience had a significant effect relative to the wrist snap/ball release component, F (2,47)=105.96, p < .0001. Understanding the processes of movement execution in underarm throwing and windmill softball pitching may facilitate developmental changes of these motor skills by utilizing the critical features and the age- and skill- related characteristics identified in this study.Keyword(s): early childhood, performance, technique