The objective of the present work is to analyse the prevalence of cyber-aggression and cyber-victimization among adolescents in Asturias (Spain) and to identify possible gender differences. To this end, 3,175 adolescents aged 12 to 18 years were randomly selected from the student population attending compulsory secondary education in Asturias and assessed. They completed three self-reported tests: an ad hoc questionnaire on sociodemographic data and communication technologies management; the "Cyber-aggression Questionnaire for Adolescents" (CYBA), to assess how frequently adolescents acknowledge having exercised various cyber-aggressive behaviours in the previous three months; and the "Cyber-victimization Questionnaire for Adolescents" (CYVIC), to assess how frequently adolescents acknowledge having been a victim of various types of cyber-aggression in the previous three months. The results obtained show a high variation in prevalence based on the type of cyber-aggression or cyber-victimization analysed. verbal cyber-aggression and online exclusion are more common than impersonation and visual cyber-aggression. There are generally no statistically significant differences between boys and girls. When differences do appear, boys generally tend to be more aggressive than girls, while girls are more likely to be victims. However, these differences are either small or very small. The implications of these results for future research and educational treatment of the problem are discussed.