The rapid development and propagation of technologies created a new context for human behaviour, with over 95% of young people below age 24 using the Internet on a daily basis. In this context, interpersonal interactions are now frequently established and maintained in faceto- face situations and in cyberspace. An important part of young people¿s everyday activities now takes place through electronic devices and the online world seems to be as real as the face-to-face world. For most young people, there is an overlap and interaction between their face-to-face and online reality, although there are some aspects of life that only take place in one of these contexts. Both antisocial and prosocial behaviours are now expressed online and offline. Even though this online and offline interaction and overlap are crucial in young people¿s lives, life-course developmental theories and research have not included both aspects yet. Thus, this project will be conducted to analyse, understand and explain an overlap and interaction among adolescent antisocial behaviours online and offline, from a developmental and dynamic perspective, including stability and change in time and longitudinal online and offline risk and protective factors. Based on several theoretical perspectives and previous empirical research, we will discover interactions and overlap between bullying, extremism, dating violence and substance use online and offline, using a between-individual and a within-individual perspectives. This will be done through five different working packages (WP). WP1 will consist of a series of systematic reviews and meta-analyses focused on an overlap and interaction between online and offline antisocial behaviours and their risk and protective factors. WP2 will consist of a cross-sectional study focused on adolescent antisocial behaviours online and offline, together with school-, family-, peer- and individual-level correlates of these behaviours. WP2 will be conducted with a survey that will be answered by a representative sample of adolescents in Cordoba and province of Cordoba. This will include around 15-20 secondary education schools with around 2,000 students in grades 1 to 4 (i.e. all the compulsory secondary education years). WP3 will consist of a prospective longitudinal study with a four-year follow up and four waves of data collection. Grade 1 participants (around 12 years old) surveyed in the WP2 will be surveyed again throughout their secondary education years in Grade 2 (wave 2), Grade 3 (wave 3) and Grade 4 (wave 4). This sample will include around 1,000 adolescents (around 50% males and 50% females), data will be collected at the beginning of each school year. WP4 will integrate findings from WP1, WP2 and WP3 in a comprehensive developmental theory of online and offline antisocial behaviour. This theory will include risk and protective factors for the initiation of the antisocial online/offline career, factors for stability and change in antisocial behaviours, and factors for persistence and desistence. It will also include designing a prevention and intervention strategy based on the findings of this project. A new taxonomy of online and offline antisocial careers will be created. Findings are expected to have an important impact on research, policy and practice and they will be widely disseminated among different stakeholders.