The purpose of this work was to model carbide precipitation in steels of a quaternary system which includes two substitutional elements. The work focuses on secondary hardening steels which are used for high-strength components, where hydrogen embrittlement is one of the major factors responsible for failure. It is believed that carbide particles can act as hydrogen trapping sites, thus reducing the risk of embrittlement. The thesis begins with a review of the physical metallurgy of secondary hardening steels and the phenomena of hydrogen embrittlement and hydrogen trapping. The basic theory for the precipitation processes, including the nucleation and diffusion-controlled growth of particles, is reviewed in Chapter 2. Significant progress has recently been made in modelling the overall kinetics of transformations which occur simultaneously. The new theory, also reviewed, adopts classical nucleation and diffusion-controlled growth concepts and takes into account the capillarity effect. In the present work, a modified model has been developed for the precipitation of needle-shaped carbides, and a new model for plate-shaped carbides. The models are then verified experimentally, using five steels designed specifically for this purpose. Using the chemical compositions of the steels and thermodynamic data, the carbide precipitation, dissolution and coarsening kinetics at 600 μC were estimated. It is found that reasonable agreement can be obtained between experiment and theory for ternary steels, when multicomponent diffusion and capillarity effects are taken into account. This applies to both needle and plate-shaped particles. The same approach was then used successfully for quaternary steels. For the specific steels studied, M2C- and M4C3-type carbides are expected to be hydrogen trapping sites which improve the hydrogen embrittlement properties. Experimental results on the hydrogen trapping capacity of the steels confirm this expectation and the relationships between the hydrogen trapping capacity and the features of M4C3 carbide particles are discussed. Finally, conclusions are drawn and suggestions are made for future work.