The present study aimed to clarify the nature and dimensions of achievement goals and to examine structural differences in students' goals across school levels. Participants were 134 students from 5th and 6th grades, and 423 students from 7th to 9th grades. A variety of achievement goals were assessed, including mastery goals and several performance-related goals representing three main dimensions: competition, self-presentation, and valence. Two alternative models were tested, using confirmatory factor analysis. For middle-school students a three factor model with presentation, competition, and simple evaluation/mastery goals, was found χ²(132, N = 134) = 160.9, p < .001; CFI = .94; RMSEA = .04, 95%CI [.02 - .06]. In the junior-high sample, one avoidance factor, one competition factor, and a simple evaluation/mastery factor, best fitted the data χ²(114, N = 423) = 269.8638 p < .001; CFI = .93; RMSEA = .06, 95%CI [.05 - .07] thus suggesting that distinct dimensions organize younger and older students' motivation. However, common to both grade levels was the existence of (a) separate but low incidence competition goals, and (b) simple evaluation goals, which encompass neither self-presentation nor competition, and are closely linked to mastery goals. Moreover, significant differences were found in the relative importance attached by students to the different types of goals (p < .001 for all comparisons), both at middle-school F(2, 266) = 220.98; p < .001; η2 = .624) and at junior-high school F(2, 820) = 464.4; p < .001; η2 = .531.