Neurotoxicity of anaesthetics in developing brain cells is well documented in preclinical studies, yet results are conflicting in humans. The use of many and different outcome measures in human studies may contribute to this disagreement. We conducted a systematic review to identify all measures used to assess long-term neurocognitive outcomes following general anaesthesia (GA) and surgery in children. The quality of studies was assessed according to the Newcastle-Ottawa Scale (NOS) for observational studies. PubMed/MEDLINE, EMBASE, Cinahl, Web of Science, and the Cochrane Library were searched for studies investigating neurocognitive outcome after GA in children <18 yr. Sixty-seven studies were identified from 19 countries during 1990–2017. Most assessments were performed within cognition, sensory-motor development, academic achievement or neuropsychological diagnosis. Few studies assessed other outcomes (magnetic resonance imaging, serum-biomarkers, mortality, neurological examination, measurement of head circumference, impairment of vision). Rating according to the NOS rewarded a mean of six stars out of nine. Some concerns prevail regarding potential inter-rater variability because of equivocal description of rating criteria. Specific features such as stability over lifetime and inter-relations of outcomes (e.g. prediction of subsequent development or diagnosis of neuropsychological conditions) are discussed. The importance of validity and reliability of the various test instruments are described. The studies vary immensely in important characteristics. Future observational studies should be more consistent in the choice of study population, age at exposure, follow-up, indication for and type of surgery, and outcomes. Assessment of sensory-motor development seems feasible in young children (age <4 yr), intelligence/cognition in older children.