The purposes of this study were to assess secondary school students' decision-making and game-play ability and to investigate how game understanding, assessed by a standardized video-based test, corresponds to students' decision-making and skill execution ability in actual game play. Students (12, aged 14-15 years) participated in a video-based game understanding test and played three different types of three vs three modified soccer games for ten minutes. All matches were video recorded and analysed using a coding instrument developed for examining the components of game play decision-making and skill execution. The results revealed that the students already had a basic conception of both offensive and defensive game situations. Furthermore, the students' game understanding and game-play ability were found to be related. (Contains 2 figures and 8 tables.)