IMPORTANCEBecause children in a preverbal stage of development are unable to voice their feelings, they completely depend on their caregiving team for the interpretation and management of their pain and discomfort. Thus, accurately validated scales to assess pain and sedation levels are crucial.OBJECTIVE To provide clinicians a complete overview on the validity and reliability of the existing pain and sedation scales for different target populations (preterm infants, term infants, and toddlers) and in different clinical contexts.EVIDENCE REVIEW BIOSIS Previews, Cumulative Index to Nursing and Allied Health Literature, Cochrane Central Register of Controlled Trials, Cochrane Database of Systematic Reviews, Embase, MEDLINE, PsycCRITIQUES, PsycINFO, PSYNDEXplus Literature and Audiovisual Media, and PSYNDEXplus Tests were the databases screened from their inception to August 2018. All studies examining the validity or reliability of a given pain or sedation scale for patients in a preverbal stage of development were included in this systematic review. Those scales that were tested for at least construct validity, internal consistency, and interrater reliability were subsequently scored using the consensus-based standards for the selection of health measurement instruments (COSMIN) checklist.FINDINGS In total, 89 validation articles comprising 65 scales were included. Fifty-seven scales (88%) were useful for assessing pain, 13 scales (20%) for assessing sedation, and 4 scales (6%) for assessing both conditions. Forty-two (65%) were behavioral scales, and 23 (35%) were multidimensional scales. Eleven scales (17%) were validated for infants on mechanical ventilation. Thirty-seven scales (57%) were validated for preterm infants, 24 scales (37%) for term and preterm infants, 7 scales (11%) for term-born children, 7 scales (11%) for preterm infants, term infants, and toddlers, and 17 scales (26%) for term infants and toddlers. Twenty-eight scales (43%) considered construct validity, internal consistency, and interrater reliability.CONCLUSIONS AND RELEVANCE Clinicians should consider using scales that are validated for at least construct validity, internal consistency, and interrater reliability, combining this information with the population of interest and the construct the scale is intended to measure.