<p>Hardened tool Steels (HS) are pivotal in Plastic Injection Moulding (PIM) due to their exceptional wear resistance, hardness, and durability under demanding industrial conditions. These materials are crucial in manufacturing high-precision moulds and complex metal components, where stringent mechanical properties and surface quality are required. This review explores the properties and applications of a broad range of Hardened tool Steels widely utilised in High-Pressure Injection Moulding (HPIM), focusing on understanding these mould steels’ thermal stability, machinability, and surface integrity under different milling strategies. Milling Hardened tool Steels presents unique challenges, particularly in maintaining Tool Wear (TW) resistance and ensuring optimal surface finishes. An examination will be conducted on recent advancements in tool steel machining, such as innovations in end-mill materials and advanced coatings. Some advanced multilayer Physical Vapour Deposition (PVD) coatings have significantly improved cutting tool performance, particularly in enhancing Tool Wear resistance and extending the overall lifespan of cutting tools used in High-Speed Milling (HSM) operations. The review will also highlight the importance of optimising machining environments, including cooling and lubricating systems used by other researchers, which can significantly reduce Tool Wear, improve machinability, and enhance the efficiency of manufacturing processes, discovering the prospects for future work and contributing to the endeavours of the injection mould industry. The careful selection of end-mill materials and machining conditions is critical for achieving precision, reducing costs, and extending Tool Life (TL) in injection moulding applications.</p>