The title of this thesis contains terms whose importance and complexity necessitated their critical interpretation and analysis of their theoretical sources. This has been accompanied by an empirical study. Leisure time is not a homogenous behavioural category and there is little consistency in its categorization and definition (Godbey, 2003; Previšić, 2000). However, despite the lack of precision in these numerous definitions, they all concur that the term ‘leisure time’ indicates the time that remains after all obligations have been completed, and that there are various options for spending that time and various influences affecting how it is spent. During our leisure time we engage in activities that we choose ourselves, which are optional, and exclude time spent in school, time spent doing household chores and time spent at work (Byrne, Nixon, Mayock and Whyte, 2006; Previšić, 2000). The features of leisure time and characteristics of adolescents imply their manifold mutual interactions, and for this reason leisure time as an everyday phenomenon is often examined through its correlation with adolescents (Ilišin, 2007). The importance of leisure time for the development of children and adolescents is becoming increasingly evident due to the possibilities for socialization and interactions offered by leisure-time activities. More than any other age group, young people are susceptible to trends (Roberts, 1997), and thus they rapidly accepted the Internet and online social networks and integrated them into all segments of their lives, which also implies the majority of their leisure time. This modernday phenomenon has changed the mode of communication within all factors of socialization, and the entire culture of leisure activities (Tapscott, 2008). The quality of life of adolescent population is a very complex phenomenon, which can be analysed from various points of view. It consists of numerous and diverse specific features that pertain to various fields of life. The quality of life is determined by certain external factors (economical, physical, environmental, safety-related), the community’s social organization, affiliation with a social group, and a number of other circumstances (Bouillet, 2006). Current research into the quality of life encompasses a wide range of disciplines, such as psychology, sociology, economics, politics, marketing, physical planning and medical sciences, but there is still no consensus regarding its precise pedagogical definition. Research into adolescents’ leisure time and their quality of life carried out to date has demonstrated that leisure time has crucial importance for the development of young people, who construct their own identities (Larsen, McGraw and Cacioppo, 2001) and their own life styles (Caldwell, Baldwin, Walls and Smith, 2004) in their leisure time, sometimes going beyond the boundaries of conventional society. From this it follows that leisure time is vital to the quality of life (Iso-Ahola, 1980). With a view to the fact that quality of life of adolescents correlates directly to the way in which they spend their leisure time (Perasović and Bartoluci, 2008; Wang and Kao, 2006), and that the Internet and social networks have become a dominant activity in adolescents’ leisure time, the aim of the thesis was to explore the relationship between their leisure-time activities and opinions on social networks, as well as their self-assessment of the quality of their lives. The first part of the thesis presents some basic features of the age group of higher secondary-school students who are in the developmental stage of adolescence. The main features of early, middle and late adolescence are pointed out, as well as the nature of their interrelations. Today’s adolescents are often described as Generation Z (Hamill, 2005); the main characteristics of this generation are presented in this chapter, which relies on knowledge from the fields of developmental psychology, psychology of adolescence and generational theory. The second part of the thesis discusses the phenomenon of leisure time from the standpoint of sociological and pedagogical analyses, in historical and social contexts. It presents a number of definitions of leisure time, its character and functions. Particular emphasis is placed on adolescents’ leisure time and various factors that influence the way they spend it. The relationship between modern technologies and adolescents’ leisure time is analysed, together as are the changes in the manner in which adolescents spend their leisure time under the influence of digital technologies, the Internet and social networks. The third chapter deals with the social network paradigm. It describes the emergence of the Internet – today’s most popular medium thanks to its interactive nature and digital background. Background information on the creation of social networks, their evolution and pertinent characteristics is provided. The methods for accessing and use of social networks are described, with particular emphasis on the ways and reasons social networks are used by higher secondary-school students. The chapter also presents recent research on the influence of social networks on adolescents’ behaviour. This section relies on media theory, media criticism and media pedagogy. The fourth chapter consists of an overview of the historical evolution of the quality of life and various ways in which it has been defined. It discusses various theories and models, indicators and possible measures of the quality of life. Particular attention is dedicated to adolescents’ quality of life, and the relationship between their leisure time and the quality of their lives. The fifth chapter is the empirical component. The goal of the study was to examine and explain the features and predictive role of leisure time and social networks in the self-assessment of the quality of life by higher secondary-school students. The initial assumptions were that the demographic characteristics of the respondents and their leisure-time activities are statistically significant predictors of the students' quality of life, that there is no statistically significant difference in the self-assessment of the quality of life based on gender, domicile and the type of school students attend, and that there is no difference in the frequency of use of social networks based on the students' gender. The study was conducted on a sample of students specializing in various subjects (N = 594), who live in both urban and rural communities of Zagreb and its surroundings. Of this sample, 23 students were interviewed. Data were collected using semi-structured interviews and grounded theory, and also using suitable instruments which, besides demographic data, included Leisure Time Scale of Activity, Scale of Opinions on Social Networks and Quality of Life Scale. The data obtained by the survey were entered in an SPSS file. All types of statistical analyses were conducted using this SPSS file, while the data obtained from the interviews were analysed using the grounded-theory method (Glaser and Strauss, 1967; Strauss and Corbin, 1998). After presentation of the results, the discussion focuses on significant data and their comparison to results of prior studies. It has been shown that there is a positive and statistically significant correlation between the self-assessment of quality of life, age, gender, leisure-time activities and opinions on social networks. Leisure-time activities have the strongest impact on the quality of life. Gender is also a statistically significant predictor of the students’ quality of life. Their social-networks attitude has not proven to be a statistically significant variable in the quantitative component of the study, but participants in the qualitative component of the study were of the opinion social networks had a considerable impact on the quality of life. There is also a statistically significant correlation between gender and time spent daily on social networks. The final chapter brings the conclusion in which the most important findings are cited and the scientific relevance of this research is underscored. The main conclusion is that selfassessment of the quality of life of higher secondary-school students is determined by their satisfaction with their leisure-time activities and their use of social networks as one of the elements of leisure time. The research results will contribute to a theoretical (re)definition of the quality of life as a complex category determined by objective and subjective circumstances in which students live and activities they engage in.