Background: A considerable proportion of secondary school students gets additional support from private tutoring lessons. Empirical findings on the effectiveness of private tutoring are inconclusive. The instructional quality of the tutoring lessons might influence their effectiveness. Aims: We analysed (1) whether students privately tutored in the subjects German or mathematics outperformed their non-tutored counterparts; (2) whether the instructional quality of the tutoring lessons can be described with the three dimensions, structure, challenge, and support; and (3) the effects of these quality dimensions on the students' academic achievement and their global satisfaction with their school and family situation. Sample: We used data from the German National Educational Panel Study (NEPS). From a total sample of N = 11,358 10th-grade secondary school students, subsamples provided information about tutoring quality in mathematics (n[subscript M] = 1,714), German (n[subscript G] = 148), and English (n[subscript E] = 490). Methods: The model of three correlated dimensions of tutoring quality was evaluated with confirmatory factor analyses, and the effects on student outcomes were tested in a structural equation modelling framework. Results: Analyses revealed neither global effects of private tutoring nor effects of its instructional quality on students' grades in mathematics or German. However, the support dimension was positively related to students' satisfaction with their school situation. Conclusions: Private tutoring cannot be recommended as a generally effective strategy to improve academic achievement in school, but might be helpful to relieve the stress level induced by insufficient achievement in school.