Introduction: Career maturity is one of the key constructs in the field of career behaviour; however, it is rarely used in the context of career counselling. Previous research of career maturity is based on a career development approach and has its origins in the work of Donald Super and his Theory of Vocational Development (Super, 1953; 1990). The main premise of Super theory is the idea that professional development can be tracked in stages, as can the overall development of the individual. According to Super (1990), each stage of career development involves one or more professional tasks or challenges that an individual must successfully overcome in order to advance in their career development. The concept of career maturity is a key construct in the Theory of Vocational Development and refers to the readiness of an individual to cope with professional developmental tasks that are related to a certain phase of career development (Super, 1953; 1990). On the basic postulates of Super's theory, the Career Construction Theory was created (Savickas, 2002), according to which career construction is stimulated by the answers given by an individual to professional development tasks, and career maturity is defined as the level of development achieved by an individual. The central part of career development according to these two developmental theories takes place in the research phase (14-24 years) which includes the developmental tasks of crystallization, specification and implementation of professional choice (Super et al., 1996). With this research, we followed the career development of adolescents who are currently in the research phase from the last grade of high school to one year after graduating from high school when individuals had to implement their professional choices and successfully enrol in the desired study. We measured the success of the tasks of the research phase with various measures of career maturity that are based on career development theories. We wanted to make an additional contribution to the definition of career maturity by combining it with a Trait-Factor approach to career, which sees the occupational choice as a single decision at a point in time. Holland's theory of career choice is the best-known Trait-Factor theory, and we used this research to examine the theoretical assumption that measures of differentiation and consistency of vocational interests can be considered measures of career maturity (Holland, 1985). According to development theories, high career maturity in the research phase should result in a successful transition to the next stages, i.e. in the case of our sample a successful transition to university. Career maturity should be associated with professional outcomes such as quality of career choice, implementation of choices, job satisfaction, and successful work behaviour (Betz, 1988). The assumption is that individuals with higher career maturity will have more successful careers because their expectations of the world of work are more realistic and useful in career planning. The main theoretical contribution of this paper is in testing the assumption of developmental theories that career maturity in high school will result in a successful transition to university, which we achieved by longitudinal monitoring of students who were in the process of transition to study. Transition outcomes were measured by various operationalization’s of motivation during the first year of study and study success. According to developmental theories, people with a high level of career maturity possess more psychosocial resources that allow them to adapt to career demands and changes which should have a positive effect on their motivation and performance in academic tasks (Savickas, 2013). Although career development according to development theories is a lifelong process, the education system in Croatia requires students to make important professional decisions at a specific time - such as enrolment after graduation. For this reason, we considered it important to examine whether career maturity in high school would result in congruence (fit of the individual and the study environment) that is considered a fundamental successful outcome of a Trait-Factor approach to career. We hypothesized that an individual’s high career maturity will be associated with good professional decisions that will result in enrolment in a study that is congruent with the individual’s interests. Given that according to Holland's theory (Holland, 1985), congruence of person and environment leads to greater motivation for work and work performance, we assumed that higher career maturity will be reflected in greater motivation during studies and success in studies and that congruence will mediate the relationship of motivation and success in study and career maturity. Methodology: This research is part of the project "Vocational Development in Adolescence: Setting the Adolescent Career Transition Model, HRZZ-1229" which lasted from July 2014 to June 2017. The participants in the research were high school students in the final grade of high school in Croatia, whose careers we followed in the next year and a half in which they make and implement a professional decision on further education or employment. The research was conducted online through the platform www.putkarijere.hr. In the first wave of research, conducted at the beginning of the final school year in November 2014, we applied measure of vocational interests (N=981). At the end of the final school year in April 2015, after students had to make a decision about future education or employment, we conducted a second wave of research in which we applied measures of career maturity (N=568). The invitation to the participants in the third wave of research was sent via email, and questionnaires of motivation during studies and success in studies were applied in April 2016 when students made the transition after graduating from high school and already attended the second semester of university (N=321). As measures of career maturity in the research we used Student Career Construction Inventory (Savickas & Porfeli, 2012), Career adapt-abilities scale (Savickas & Porfeli, 2012) and The Career Decision Self-Efficancy Scale (Taylor & Betz, 1983). As a measure of vocational interests, we used a short version of the Personal Globe Inventory (Tracey, 2010). Based on the results on six dimensions of interest, we calculated measures of differentiation and consistency of vocational interests. In the research, we used some traditional but also new measures of differentiation and consistency based on the cosine function that use the whole profile of the individual's interest. For the purpose of calculating congruence measures, study programs are coded on the basis of the International Standard Classification of Occupations, and results in RIASEC dimensions are added to each study. Congruence measures are based on the RIASEC results obtained by the PGI questionnaire in the first wave and the estimates of RIASEC results for study environments, and are operationalized through traditional indices and new measures based on Euclidean distance proposed by Tracey and Robbins (2006). In order to measure motivation for study, we applied the Work engagement scale (Schaufeli and Bakker, 2003) and the Basic Psychological Needs Scale (Deci & Ryan, 2000).We measured the success of the study through the grade point average, In-role Performance (Williams & Anderson, 1991), and the Counterproductive work behaviour questionnaire for student population (Tomšić, Jerneić & Tonković Grabovac, 2014). Results: Problem 1. Examine the relationship between measures of career maturity and the differentiation and consistency of vocational interests in order to determine whether measures of the structure of professional interests can also be considered measures of career maturity. The correlations of career maturity with the differentiation are positive and low. The FirstLast Index and Amplitude have systematically low correlations with all three measures of career maturity. The correlations of career maturity with consistency are insignificant with the exception of the correlation of the conventional index with the Student Career Construction Inventory which is on the verge of significance. In conclusion, based on the analysed correlations and very low correlation of career maturity with differentiation and non-existent consistent correlation with consistency of vocational interests, we can conclude that indicators of the structure of interests cannot be considered measures of career maturity as defined by development theories. We also verified this conclusion by exploratory factor analysis that extracted the independent factors of these constructs. Measures based on developmental theories should be used as measures of career maturity because the are based on the essential definition of career maturity as a measure of the degree of fulfillment of the tasks of the research phase. Problem 2. Examine the relationship between measures of career maturity in high school with motivation during studies and success in studies during the first year of university. Measures of career maturity measured in high school are generally moderately related to study outcomes. All measures of career maturity are positively associated with study engagement. Career adapt-abilities had the greatest contribution in explaining study engagement. Career maturity is positively related to the degree of satisfaction of basic psychological needs, mostly with the need for competence. The best predictor of psychological needs was career decision selfefficacy. Grade point average as a measure of academic performance was not associated with any measure of career maturity in high school. The grade point average is the only indicator of the transition outcome that was not related to career maturity. The other two measures of success in the study showed the expected relationships with career maturity. In-role Performance was positively associated with all measures of career maturity, most notably with career adapt-abilities. Contraproductive academic behaviour is negatively associated with all measures of career maturity, and again mostly with career adapt-abilities. Problem 3. Does the congruence of interests and study environments mediate the relationship between career maturity and outcomes in the study. No overall score on career maturity measures was associated with congruence measures. Congruence measured via Euclidean distance was associated with some study outcomes: study engagement, grade point average, and in-role performance. We tested the mediating role of congruence in relation to career maturity for three subscales of career maturity (focus, selfawareness, and choice of professional goals) that were significantly related to congruence. We can say that for subscales that had a clear relationship with congruence, congruence systematically represents a mediator of the relationship of career maturity and study engagement. Conclusion: This research confirmed the importance of career maturity in high school for a successful transition to university. Measuring and developing career maturity should be an important part of career counselling in secondary schools. Measures based on developmental theories that examine the fulfilment of the tasks of the research phase should be used as measures of career maturity. Differentiation and consistency of vocational interests cannot be considered as measures of career maturity as defined by development theories. Students who had a higher level of career maturity during high school had greater motivation during their studies, which is reflected in the positive relationship of career maturity with study engagement and satisfaction of basic psychological needs. Career maturity did not prove to be a good predictor of grade point average, but it was a good predictor of success in work assignments and contra productive academic behaviour. The congruence of interests and the study environment did not prove to be a clear mediator of the relationship between career maturity and study outcomes.