Objective: To assess the convergent validity and reliability of joint angle measurements from a new video goniometer iPhone/iPad application separately in adults, older and young children. Design: Cross-sectional. Setting: Child care and university environments. Participants: Fifty-four adults (mean SD=22.5 [plus or minus] 4.5y), 20 older children (mean [plus or minus] SD=10.9 [plus or minus] 2.2y), 20 younger children (mean [plus or minus] SD=1.6 [plus or minus] 0.8y) (N=94). Interventions: Adults and older children performed both standardized static positions and functional activities. Younger children performed only a functional activity protocol. Main Outcome Measures: Joint angle measurements using the app were validated against a commercially validated two-dimensional goniometric software program. In addition, validity of the app was compared to a standard mechanical goniometer for the measurement of angles drawn on a white board. Intra- and interrater reliability were assessed through independent rescoring of videos. Results: Correlations between joint angle estimates obtained from the app and goniometer software or a mechanical goniometer were positive and very strong (r>0.900; P<0.0001). The intraclass correlation coefficient (ICC) for repeated scoring from the app indicated excellent intra- and interrater reliability (ICC>0.900). Conclusions: High correlations for repeated measures and comparison to gold standard angle measurement instruments suggest that the new app is a valid and reliable tool for assessing joint angles during functional activity. This tool may provide clinicians an inexpensive yet accurate method for quantification of movements and immediate feedback on range of motion during tasks in a natural environment. [This is the in press version of an article published in "Archives of Physical Medicine and Rehabilitation."]