Partial discharge (PD) is a localized electrical discharge that only partially bridges the insulation between conductors. It usually occurs in gas-filled voids or along insulation surfaces when the local field exceeds the breakdown strength. Each pulse is tiny, measured in picocoulombs (pC), but repeated discharges gradually erode the insulation until it fails. The main types are internal, surface, corona, and electrical treeing. Common causes include manufacturing and installation defects, insulation ageing, moisture, and contamination. PD is measured by the conventional electrical method of IEC 60270, or by non-conventional methods such as HFCT, UHF, acoustic emission, and dissolved gas analysis (DGA). The phase-resolved PD (PRPD) pattern helps identify the defect type. Research now focuses on continuous online monitoring and artificial intelligence for pattern classification and insulation life estimation. Early PD detection shifts maintenance from reactive to predictive, extending the life of transformers and cables and improving grid reliability. ​Al-Mustaqbal University — The First University in Iraq.