Although all youngsters with neurodevelopmental behavioral disorders (NDBDs) are at increased risk to develop problems, wide variability exists. Some children show extreme problems, whereas others thrive relatively well. This project aims to examine how temperament and parenting, vitally crucial for all children, can explain this wide heterogeneity in youngsters with NDBDs. To unravel syndrome-(a)specificity, I will target differences across typical controls and four prevalent NDBDs: Autism Spectrum Disorder, Attention-Deficit/Hyperactivity Disorder, Down Syndrome, and Cerebral Palsy. First, I will address the diagnostic relevance of temperament and parenting, describing the salient differences in each group. These syndrome-specific differences will be revealed through meta-analytical integrations of the trait literature (Study 1) and fine-grained empirical comparisons of both temperament and parenting (Study 2). Second, I will address the predictive value of the temperament-parenting interplay to explain (mal)adaptive heterogeneity. Fundamentally, I expect that these predictive processes will be highly similar across youngsters with and without NDBDs. This hypothesis will be tested cross-sectionally for all four NDBDs (Study 3) and prospectively for youngsters with autism and ADHD (Study 4). Results of my project will expand understanding of the behavioral phenotype in each group. Also, it will help paving the paths towards less problems and more well-being in youth with NDBDs.