This systematic review explores the topic of Part C Early Intervention (EI) dosage reporting and decision-making factors. From an extensive search following PRISMA guidelines, we identified 14 interdisciplinary studies (combination of descriptive, correlational, mixed method, qualitative, and single case design) that met the criteria and synthesized them based on the reported definitions and data of EI dosage components, including frequency, duration, and intensity, as well as factors associated with this data at child, family, and community levels. Results indicate frequency was most reported, whereas duration and intensity were inconsistently reported, as were service delivery models. The inconsistency of reporting did not allow for interpretations of effective dosage; however, disparities based on different factors are shared. Results and discussion are provided for each level of factors, such as family demographics, insurance enrollment, and disability classification. Implications for further research and practice, especially for EI dosage decision-making and reporting, are included.