Since Bowlby (1958, cited in Bowlby, 1969) originally introduced the theory of attachment it has been written about extensively and a vast amount of research has contributed to the development of the theory. In more recent years research has focused on the possible link between attachment and psychopathology. The major aim of the present meta-analysis was to contribute to this research effort by establishing the magnitude of the effect size for the relationship between attachment security and internalizing psychopathology; and attachment security and externalizing psychopathology, in children and adolescents. Four separate meta-analyses were conducted investigating internalizing and externalizing problems in cross-sectional and prospective studies. A comprehensive literature search was conducted to identify relevant studies for inclusion in the analysis. Identified studies were assessed for eligibility according to stringent inclusion and exclusion criteria. A total of 23 studies contributing 45 effect size correlations, involving 3793 different participants were considered eligible for inclusion. Relevant information was extracted and coded from the studies before the analyses were conducted. For cross-sectional studies the mean effect size correlation for attachment security and internalizing psychopathology was r = -0.24 (k = 14; p <0.01; 95% CI = -0.31, -0.17). For attachment security and externalizing psychopathology the mean effect size was r = -0.28 (k = 16; p <0.01; 95% CI = -0.34, -0.21). In terms of prospective studies the mean effect size correlation for attachment security and internalizing psychopathology was r = -0.17 (k = 8; p = 0.01; 95% CI = -0.28, -0.04); and for externalizing psychopathology it was r = -0.09 (k = 7; p = 0.02; 95% CI = -0.16, -0.01). When attachment security and psychopathology were measured concurrently, there was evidence of a negative association for both internalizing and externalizing psychopathology. Although the magnitude of effect was smaller for prospective studies evidence was also found for the predictive validity of a lower level of attachment security in the development of both internalizing and externalizing psychopathology. Theoretical explanations for these findings are presented and the research and clinical implications are discussed in terms of the limitations of the study.