Wellhead systems functioning under high pressure and high temperature involve a situation in which pressure containment, thermal expansion, sealing performance, materials compatibility, and observance of operational procedures all become closely interrelated. This review has gathered the most up-to-date literature published between 2020 and 2025 on the reliability, integrity and risk management of wellheads and of the other pressure-control barriers used in complex upstream operations. It examines the load paths from the casing and tubing all the way to the casing heads, tubing heads, spools, connectors, valves, seals and the Christmas-tree assemblies; the various types of degradation caused by pressure-temperature cycling, corrosion, erosion, fatigue, wellhead movement and the deterioration of the seals; and the increasing significance of continuous surveillance, digital twins and risk-based maintenance. A structured integrative review method was used, with clear eligibility criteria and in accordance with the PRISMA 2020 reporting framework, since a meta-analysis was not carried out as the body of evidence includes experiments, numerical models, field case studies, standards and review articles. The synthesis demonstrates that the reliability of HPHT systems is not mainly a question of the rating of any one component but rather results from the way in which design margins, transient loads, material-environment compatibility, the quality of installation and aging over time interact. The paper suggests a lifecycle integrity framework which connects qualification, barrier verification, condition monitoring, anomaly diagnosis, risk ranking, intervention planning and feedback. It concludes that in the future HPHT assurance should focus on representative full-envelope qualification, coupled thermo-mechanical models, health indicators for seals and connections, validated digital-twin decision support, and risk controls which remain effective even in the presence of uncertainty.