This special issue focusing on pediatric psychology research in young children highlights 15 papers that focus on infants through preschoolers. Studies selected for inclusion cover a range of pediatric health conditions such as food allergy, medical trauma, injuries/traumatic brain injury, cancer, inflammatory bowel disease, pain, and sleep. The inherent challenges of researching young children are described, and studies vary in their methods for assessment and intervention; multiple studies include an observational component or developmental evaluation. Six of the studies employ diverse samples of children and/or parents and demonstrate the feasibility as well as importance of increasing our understanding of factors related to health disparities. Taken together, the special issue demonstrates the high quality of research focusing on young children. As the special issue editors, we hope this collection will spark an interest in others to focus on research with young children across pediatric populations.