This master's thesis presents a comprehensive investigation and prediction of crucial toughness-related parameters in 15-5 PH steel, a widely used precipitation-hardening stainless-steel alloy known for its excellent combination of strength and toughness. The parameters studied are ductile-brittle transition temperature (DBTT) and fracture toughness (JIC), both of which play a critical role in determining the material's performance under various environmental conditions and loading scenarios, especially in applications such as aerospace. This work shows promising results, with the DBTT prediction having an impressive coefficient of determination of 0.916 over the temperature range of -90 to -12 °C. In addition, the JIC prediction has a notable coefficient of determination of 0.906 in the range of 200 to 450 kJ/m2. These accurate predictions are critical for the application of 15-5 PH steel in various fields, as they provide the ability to estimate the critical toughness properties of the material without the need for extensive experimental testing. Furthermore, this research highlights the intricate relationship between microstructural features and yield strength as well as their influence on the toughness parameters of the material. By developing predictive models that provide such a high level of accuracy, this work not only contributes to the field of physical metallurgy but also improves our understanding of material behaviour, ultimately increasing the reliability and versatility of 15-5 PH steel for a wide range of applications.