This study proposes the development and validation of the Neurodevelopmental Disorders Ecological Tracker (N-DET), a novel caregiver-report Ambulatory Assessment tool designed to measure changes in children with neurodevelopmental disorders (NDDs). The N-DET aims to address limitations of existing measures by capturing diverse daily manifestations of NDDs with high ecological validity and temporal resolution. It measures key areas, including challenging behaviors, repetitive restrictive behaviors, social interactions, and communication. The 24-month study will enroll 60 children aged 3-7, equally divided between autism and genetic conditions associated with neurodevelopmental disorders. Participants will complete the N-DET daily for one out of every six weeks over 18 months, resulting in 12 waves of data collection. Validation will involve comparison with standardized measures administered longitudinally and automated analysis of naturalistic audio recordings. The study’s goals include evaluating the N-DET’s content, assessing its validity against external measures, determining the design’s feasibility, and exploring patterns of change in NDD features. This innovative approach aims to establish the N-DET as a sensitive measure of change over time for NDD clinical trials and as a tool for enhancing treatment personalization, addressing the urgent need for improved outcome measures in this population.