This chapter explores and evaluates the underlying principles, delivery and future of youth justice in England and Wales in the emerging context of complexity. A reductionist risk-based approach has attained hegemony in the Youth Justice System (YJS) of England and Wales, perpetuating understandings of offending behaviour by young people as the linear, proportional and deterministic outcome of exposure to ‘risk factors’. This simplistic caricature of young people's lives has been derived from positivist ‘Risk Factor Research’ (RFR) (Case and Haines, 2009), which itself has informed a ‘Risk Factor Prevention Paradigm’ (RFPP) for youth justice. We evaluate the dominant positivist riskbased model of youth justice in relation to the inherent complexity, unpredictability, context-dependence and multidimensionality of the young people and behaviours targeted by the YJS. We put forward a post-positivist argument (rather than a critical realist stance) that quantitative criminology is both necessary and valuable, but not in the positivist form in which it has come to be applied to youth justice, such as through crude measurement and crude linear statistical tests. We argue for better measurement of social-scientific phenomena (cf Byrne, 1998) and post-positivist statistical analyses. Our posited approach is grounded in the dynamic model inherent to Complex Systems Science and has two basic elements: that the measurement of key concepts (eg ‘risk’, ‘offending’) in positivist RFR and practice has been too crude and insufficiently ‘fractal’ (following Mandelbrot, 1967); and that there has been insufficient sensitivity to initial conditions in the linear analysis of relationships between risk and offending behaviour (following Lorenz, 1963). Therefore, we are not arguing against measurement, statistical analysis and quantitative methods per se, but arguing that much quantitative criminology and RFR, in particular, has utilised measurement scales and quantitative analyses that are too crude to sustain the conclusions that many have come to. As a consequence, we suggest that the results of RFR are largely artefactual and present inaccurate and misleading accounts of causality and offending behaviour by young people. Finally, we draw on the critique developed here to assess a proposed new framework for understanding and responding to offending behaviour by young people.