Psychometric and utility studies on Social Emotional Assessment Measure (SEAM), an innovative tool for assessing and monitoring social-emotional and behavioral development in infants and toddlers with disabilities, were conducted. The Infant and Toddler SEAM intervals were the study focus, using mixed methods, including item response theory modeling and classical test theory. Results using a Rasch one-parameter logistic model indicated model fit statistics were consistent for age and item difficulty as well as for ability and item characteristics. Classical test theory analyses generally confirmed the developmental structure; mean scores increased with age and were significantly correlated across 6-month increments. Reliability studies indicated strong internal consistency and moderate interrater agreement between teachers. Test-retest reliability results completed by parents online indicated significant agreement. Overall, 90% of parents reported the SEAM asked appropriate questions and took less than 10 min to complete. (Contains 5 tables and 1 figure.)