OBJECTIVE To assess the relevance of Quantitative electroencephalography (qEEG) measures as outcomes of disease severity and progression in PD. METHODS Main databases were systematically searched (January 2018) for studies of sufficient methodological quality that examined correlations between clinical symptoms of idiopathic PD and cortical (surface) qEEG metrics. RESULTS Thirty-six out of 605 identified studied were included. Results were classified into four domains: cognition (23 studies), motor function (13 studies), effect of treatment (7 studies), and other (10 studies). In cross-sectional studies, EEG slowing correlated with global cognitive impairment and with diffuse deterioration in other domains. In longitudinal studies, decreased dominant frequency and increased θ power, reflecting EEG slowing, were biomarkers of cognitive deterioration at an individual level. Results on motor dysfunction and treatment yielded contrasting findings. Studies on functional connectivity at an individual level, longitudinal studies on other domains or on connectivity measures, were lacking. CONCLUSION QEEG parameters reflecting EEG slowing, particularly decreased dominant frequency and increased θ power, correlate with cognitive impairment and predict future cognitive deterioration. QEEG could provide reliable and widely available biomarkers for non-motor disease severity and progression in PD, potentially promoting early diagnosis of non-motor symptoms and an objective monitoring of progression. More studies are needed to clarify the role of functional connectivity and network analyses.