Fentanyl is a systemic opioid related to the phenylpiperidines, it is used in anaesthetic practice and in analgesia and the analgesic effect is about 100 times higher than that of morphine. Fentanyl is highly lipid soluble, rapidly crosses the blood-brain-barrier, and fentanyl concentrations rapidly decline in plasma and cerebrospinal fluid. Fentanyl causes respiratory depression and decreases the heart rate through vagal activation. Fentanyl may be administered intravenously, orally, by transdermal, intranasal or by buccal application and the oral bioavailability is poor. In infants, fentanyl is given for short term use, sustained use, and during therapeutic hypothermia. In children, fentanyl is given intravenously, by transdermal application, or by buccal administration and the fentanyl dose varies with the child age and body-weight. Fentanyl has been found efficacy and safe in infants and children but it may induce adverse-effects and fentanyl causes different effects in infants and children. Following intravenous administration of fentanyl to infants and children, the fentanyl elimination half-life ranges from 208 to 1,266 min and the distribution volume ranges from 1.92 to 15.2 L/kg. Such variability is due to the wide variation of subject’s demographic characteristics. Fentanyl interacts with drugs, the treatment and trials with fentanyl have been studied in infants and children. Fentanyl freely crosses the human placenta and poorly migrates into the breast-milk. The aim of this study is to review fentanyl dosing, efficacy, safety, effects, adverse-effects, metabolism, pharmacokinetics, drug interaction, treatment, and trials in infants and children, and fentanyl placental transfer and migration into the breast-milk.