AbstractRecently, ultrahigh strength low alloy steels, e.g. AISI 4340 and 300M, have been used increasingly for critical structural aircraft and aerospace applications. These steels can be employed successfully at yield strengths of≥1400 MN m−2but their use has often been limited in commercial practice because of low fracture toughness compared with other types of ultrahigh strength steel. The results of studies carried out over the past two decades to improve the fracture toughness are presented. Particular emphasis is placed on improvements obtained by microstructural control via thermal and thermomechanical treatments, sulphide inclusions, and new alloying design. The major metallurgical factors controlling fracture toughness are discussed for each of these techniques.MST/1413