Aim: The aim of this study was to assess the orthodontic treatment needs of adolescents in Zagreb, Croatia, in relation to their orthodontic treatment history, caries experience and socio-demographic parameters. Methods: The study sample comprised 1, 289 adolescents from 12 randomly selected public schools in Zagreb, Croatia. The subjects were 15–18 years old (mean age 16.3 ± 1.4), and 51% of them were girls. The Dental Aesthetic Index (DAI), the sum of the numbers of decayed, missing and filled teeth (DMFT index), and a questionnaire (covering socio-demographic issues, oral health-related attitudes and behaviours) were employed in this study. The data was analyzed by means of Chi-square test, analysis of variance, and multiple logistic regression models. Results: The DMFT score was higher in adolescents with no orthodontic history (5.2 ± 3.7) than in those who were under orthodontic treatment at the time of the research (4.5 ± 3.2; p = 0.043). More than 60% of the adolescents have never undergone any orthodontic treatment, around 24% previously undergone treatment and 15% were under treatment at the time of the research. Overall, 85% of the adolescents’ orthodontic appliances were removable, and the girls were more often under orthodontic treatment. One fifth of the studied population had severe or very severe malocclusion. Adolescents with previous orthodontic treatment were more often interested in better teeth alignment, changes in their teeth positioning and continuing orthodontic treatment. Multiple logistic regression model demonstrated that previously treated adolescents, in comparison with their untreated peers, were on average older (p = 0.002), were less satisfied with the appearance of their teeth (p = 0.001), they had higher malocclusion severity (p = 0.046), and fewer dental caries (p < 0.001), changed toothbrushes more often (p = 0.012), and their mothers attained higher education (p < 0.001). Conclusions: Although many adolescents received orthodontic treatment, the severity of their malocclusion was still somewhat high, and they were more often dissatisfied with their treatment outcome. Mothers’ educational level was found to be the most important socio-demographic predictor for children’s involvement in orthodontic treatment. The caries experience increased in comparison with the Croatian national survey data from 1999.