The National Early Intervention Longitudinal Study (NEILS) is being conducted to address some of the most important questions related to the implementation and outcomes of Part C of the Individuals with Disabilities Education Act (IDEA). NEILS is following a nationally representative sample of children from birth to 3 years old and their families through and after their early intervention experiences. This report provides context and methodological background for interpreting the findings from NEILS. It describes the research questions, conceptual framework, sample, data collection instruments and procedures, and timeline for the study. (Author/SG)