Background: Pregnancies complicated by placental insufficiency are characterised by increased oxidative stress. It has been suggested that melatonin has antioxidant properties and contributes to the maintenance of placental homeostasis. Objective: To systematically review the available literature about melatonin in pregnancies complicated by placental insufficiency, specifically preeclampsia and fetal growth restriction, exploring three different aspects: 1) maternal melatonin levels; 2) expression and activity of melatonin placental receptors; 3) effects of maternal melatonin administration. Search strategy: PubMed (Medline) and Scopus were searched until December 2020. Selection criteria: Published literature in English language describing the role of melatonin in pregnancies complicated by placental insufficiency, with a specific focus on preeclampsia and fetal growth restriction. Data collection and analysis: Identified studies were screened and assessed independently by two authors. Data were extracted and compiled in a qualitative evidence synthesis. Main results: Pregnancies complicated by placental insufficiency show a significant reduction in maternal systemic serum melatonin levels and a significant reduction in the expression of placental melatonin receptors. Melatonin administration in pregnancy seems safe and may reduce oxidative stress. Conclusion: The circadian pattern of melatonin secretion seems to be altered in pregnancies complicated by placental insufficiency. This is reflected by lower production of melatonin, with consequent lower systemic and placental concentrations, and lower expression of melatonin receptors, thus reducing the local release of the indole and its autocrine function. Funding: None Keywords: Melatonin, Preeclampsia, Fetal growth restriction, Placental insufficiency, Review.