<p>Single-molecule junctions (SMJs), representing the ultimate limit of electronic device miniaturization, show fascinating quantum phenomena due to the dominance of quantum effects at this scale. Although theoretical frameworks have provided valuable insights into SMJ behaviour, the complexity of real-world molecular junctions necessitates a more comprehensive understanding of the interplay between various factors, including molecule–electrode interfaces, electron–phonon interactions, spin–orbit coupling and electron–electron correlations. This Review explores the interplay between quantum correlation effects, such as quantum interference, vibrational effects, molecular exciton behaviour on electronic transport and quantum spin phenomena through discussion of experimental breakthroughs alongside a critical analysis of the relevant theoretical models. A unified perspective on the diverse range of quantum phenomena observable in SMJs is provided, with the aim of stimulating further research and the development of novel device functionalities exploiting these effects.</p>