A range of plain carbon, carbon-m~mganese cmd low alloy cast, steels were tested in order to determine their Various fracture toughness values under elastic and elastic-plastic conditions. The main fracture toughness parameters which are considered are (1) Linear Ilastic Fracture Mechanics (LEFM), (2) the J-Contour Integral, and (3) Crack Opening Displacement (COD). Rasults are obtained from fracture toughness specimens of various dimensions and the relevance of the validity criteria to cast steels is considered in some detail, In addition, the effect of casting position on specimen toughness values was noted. Valid K1C results according to LEFM, were obtained for three of the eight cast steels tested. Although KIC values from LEFM were not; obtained from the remaining five steels, critical COD and J -integral values were determined. It is postulated that these value and particularly the critical J values can be used, with confidence ,for material selection or in defect tolerance calculations using these steels. Toughness values were found to vary with casting position in several of the steels tested and the possible reasons for such variations are discussed in the Thesis,