Objective: To develop an outcomes instrument that assesses observations that can be reliably reported by caregivers and can be used to assess health of infants with a cleft lip or cleft lip and cleft palate (CL±P) and impacts of treatments. Design: Cross-sectional, mixed methods study. Setting: Caregivers and health-care providers were recruited from 3 academic craniofacial centers and national advertisements. Most interviews were conducted by telephone, and surveys were completed online. Participants: Caregivers had a child less than 3 years of age with CL±P and spoke either English or Spanish. Health-care providers were members of a cleft team. Caregivers (n = 492) and health-care professionals (n = 75) participated in at least one component of this study. Main Outcome Measure(s): Caregivers and health-care providers participated in tasks related to instrument development: concept elicitation for items within relevant health domains, prioritization of items, and item review. Results: We identified 295 observations of infant well-being across 9 health areas. Research staff and specialists evaluated items for clarity, specificity to CL±P, and responsiveness to treatment. Caregivers and health-care providers rated the resulting list of 104 observations and developed the final instrument of 65 items. Conclusions: In this phase of development of the Infant with Clefts Observation Outcomes (iCOO) instrument, items were developed to collect caregiver observations about indicators of children’s health and well-being across multiple domains allowing for psychometric testing, sensitivity to changes associated with treatment, and documentation of the effects of treatment.