In this paper the refinement and psychometric properties of the Infant-Toddler Social and Emotional Assessment (ITSEA) are described. Results from a sociodemographically diverse birth cohort sample of 1,235 parents of children between the ages of 12 and 36 months are presented. Confirmatory factor analyses supported the hypothesized Internalizing, Externalizing, Regulatory, and Competence domains as well as the 17 individual scales that comprise the ITSEA. Findings for 3 additional indices useful in identifying significant psychopathology are presented. Subgroup analyses revealed structural invariance and expected mean level differences across both child sex and 6-month age bands. Child sex differences emerged for some problem and most competence scales, with boys rated as higher on Activity/Impulsivity and girls rated higher on Anxiety and most Competence scales. All competence scores increased across age groups. Problem behaviors showed a more mixed developmental pattern. Test-retest and interrater reliability were acceptable. Associations between the ITSEA and independent evaluator ratings and parental ratings of child behavior problems, temperament, and parental distress support the validity of the instrument.