This book addresses the question of whether or not behavioural differences between children can be most appropriately characterised by dimensions of psychological problems or by categorical diagnoses. It describes the concepts and methods that have been developed and applied within developmental psychopathology using this dimensional approach. The book reviews evidence on the interplay between genes and the environment in influencing internalising problems, externalising problems, Attention Deficit Hyperactivity Disorder (ADHD) and Autism Spectrum Disorder (ASD), and on the hierarchical factor structure underlying these behavioural dimensions. It provides an appraisal of the state of knowledge on the longer-term sequelae of these problems and on the efficacy of treatments that have been developed for them. Key areas of coverage include:  Multivariatedata analytic methods for investigating behavioural differences (e. g. , path analysis, cluster analysis, structural equation modelling, networkanalysis) and their associated theoretical frameworks (e. g. , hierarchicalfactor models). Methodsto investigate the biology of behavioural differences (e. g. , quantitativeand molecular genetics, epigenetics, and brain imaging). Thedesign of research studies that can test most directly for causality(i. e. , randomised controlled trials) and others that can estimateplausible causal relationships from associations and correlations. Reviewsof studies that have applied these methods to understand the developmentalcourse of internalising and externalising behaviours and theneurodevelopmental problems of Attention Deficit Hyperactivity Disorder(ADHD) and Autism Spectrum Disorder (ASD). ·         The problems that have arisen from nonreplication in research studies and the remedies that have been proposed, including the importance of meta-analyses.    ·         The development of international collaborations to pool data from studies in response to the realisation of the need for very large data sets in genetic and in brain-imaging studies. The strengths and weaknesses of machine learning approaches to the analysis of these large data sets and to the more amorphous Big Data. Developmental Psychopathology is an essential reference for researchers, professors, and graduate students as well as clinicians and other professionals in developmental psychology, clinical child and school psychology, child and adolescent psychiatry, paediatrics, clinical social work, public health, educational psychology, and all related disciplines.