Growing research has demonstrated that game-based approaches (GBA) have been favoured by Physical Education (PE) teachers as means to increase all students' competence at a physical and cognitive level. As one of the most popular GBA within PE, the Teaching Games for Understanding (TGfU) model offers a student centered perspective to PE teaching, one that foregrounds tactical understanding as a basic requirement for intentional and coherent skill learning. By utilizing the TGfU model, the aim of the present research was twofold: a) to present the implementation of a GBA intervention program within the context of Greek PE, and b) to assess its impact on primary school students' game performance. Four in-service PE teachers, trained in the use of TGfU, taught sixteen 45-minute invasion-game units, each being responsible for a different class of Grade three and four students, from two separate schools, in Athens, Greece (N=91, approximately n=23 per class). Four students per teacher (2 boys and 2 girls) were randomly selected from each class (n=16) and were allocated in four team units with four players each. Changes in the four units' game performance were assessed by expert judges in a four vs. four developmentally modified handball game during pre and post intervention phases of the study. Data were collected on four total game performance scores (appropriate decision making, appropriate and inappropriate skill execution, and support) with the Game Performance Assessment Instrument (GPAI) (Oslin, Mitchell, & Griffin, 1998). Reflective PE teacher journal entries involving students' and lessons' strengths and weaknesses were analysed along with GPAI scores at the end of the program. The Wilcoxon test for two dependent samples that was applied, revealed that although participants in all team units achieved better game performance scores at the end of the program, these changes were not statistically significant. According to journal entries, this finding was attributed to the size of the sample, the age of the participants and the unsupportiveness of the research context. It can be concluded that in order for students' progress to be traced in its entity within TGfU instructional contexts, both quantitative and qualitative performance indices are warranted.