Objective: To conduct a systematic review of the measures designed to assess sluggish cognitive tempo (SCT) since the first SCT scale using careful test-construction procedures was published in 2009. Methods: The MEDLINE (PubMed), Embase, PsychINFO, and Web of Science databases were searched from September 2009 through December 2019. Articles reporting on the reliability (internal consistency, test-retest, and inter-rater reliability), structural validity (an aspect of construct validity focused on items' convergent and discriminant validity), concurrent and longitudinal external validity, invariance, or intervention/experimental findings were included. Results: Seventy-six studies met full criteria for data extraction and inclusion. Nine measures for assessing SCT were identified (seven assessing parent-, teacher-, and/or self-report in children and two assessing self- and/or collateral-informant report in adults). Each measure has demonstrated acceptable to excellent reliability. All or at least the majority of SCT items on each measure also had structural validity (high loadings on an SCT factor and low loadings on an ADHD inattention factor). Studies have supported the invariance of SCT across sex and time, and there is also initial evidence of invariance across informants, ADHD and non-ADHD youth, and ADHD presentations. The Child and Adolescent Behavior Inventory (CABI), Child Concentration Inventory, Second Edition (CCI-2), and the Barkley Adult ADHD Rating Scale-IV (BAARS-IV) have particularly strong support for assessing parent/teacher-reported, youth self-reported, and adult self-reported SCT, respectively. Conclusion: The SCT measures included in this review share numerous positive properties, have promising psychometric support, and have proven useful for examining the external correlates of SCT across the life span. Although substantial progress has been made over the last decade, work remains to be done to further improve the assessment of SCT and key directions for future research are provided. [This paper was published in "Journal of the American Academy of Child and Adolescent Psychiatry" v60 p690-709 2021.]