The Face, Legs, Activity, Cry and Consolability (FLACC) scale is one of the most widely used behavioural observation pain scales. However, the psychometrics of the scale have not been adequately summarised and evaluated to provide clear recommendations regarding its use. The aim of this study was to rigorously evaluate the reliability, validity, feasibility, and utility of the scale for clinical and research purposes and provide recommendations regarding appropriate use of the scale. Databases searched were MEDLINE, CINAHL, Embase, PsycINFO (using the Ovid, PubMed, and Ebscohost platforms), The Cochrane Database of Systematic reviews and Cochrane Controlled Trials, and Google Scholar. Psychometric evaluation studies reporting feasibility, reliability, validity, or utility data for the FLACC scale applied to children (birth to 18 years) and randomised controlled trials (RCT) using the FLACC scale to measure a study outcome in infants and children. Data extraction included study design, population demographics, and psychometric data. Analysis involved in this study are quality assessment of the psychometric evaluation studies and the RCTs using the COSMIN checklist and the Jadad scale, respectively, and narrative synthesis of all results. Twenty-five psychometric evaluations studies and 52 RCTs were included. The study population, circumstances, and quality of the studies varied greatly. Sufficient data addressing postoperative pain assessment in infants and children exist. Some positive data support the psychometrics of the scale used to assess postoperative pain in children with cognitive impairment. Limited and conflicting data addressing procedural pain assessment exist. Content validity and scale feasibility have had limited psychometric evaluation. There are insufficient data to support the FLACC scale for use in all circumstances and populations to which is currently applied.