Purpose Based on the notion that a player must know what to do and how to do it in order to determine his actions in the games, the aim of current study was comparison of declarative and procedural knowledge of soccer among soccer school players with different age groups. Methods 456 players of soccer school from U-10 (95 players), U-13 (225 players) and U-16 (136) age groups completed Soccer Tactical Knowledge Test (STKT) (Serra-Olivares and Garcia-Lopez, 2016).This instrument with taking two basic pedagogical principles outlined by the game based approaches (Mitchell et al., 2006) into account, the importance of the specific vocabulary-language and the individual differences of each learner, in comparison with other tools does not only evaluate "knowing what and how to do" in terms of declarative and procedural knowledgeof the invasion games, but also "knowing why to do" in relation to the tactical principles for attacking proposed by Bayer (1992). In current study, declarative tactical knowledge (36 items)with subscales include specific knowledge on technical-tactical individual and collective elements when attacking for invasion games and roles and positions in soccer, and procedural tactical knowledge (15 items)with subscalesinclude specific knowledge in situations of keeping the ball, moving forward to the opposite goal and scoring in soccer were separately and together as total tactical knowledge assessed by STKT. Results Results showed significant differences in total, declarative and procedural tactical knowledge (F2,453= 16.54, F2,453= 15.22 and F2,453= 9.57, p= 0.000, respectively) among U-10, U-13 and U-16 groups. Bonferroni post hoc revealed these significant differences just between U-10 with U-13 and U-16 groups in total (p= 0.000, both), declarative (p=0.002 and p=0.000) and procedural (p= 0.000, both) tactical knowledge.Indeclarativeknowledge, U-13 and U-16 groups in all technical and tactical mentioned subscales were significantly better than U-10
