In this work, aerothermal investigations of a highly loaded HP turbine blade are presented. The purpose of such investigations is to improve the physical understanding of the heat transfer in separated flow regions, with the final goal of optimizing cooling configurations for aerodynamically highly loaded turbine designs. The analysis is focused on the T120 cascade, that was recently tested experimentally in the framework of the European project AITEB-2 (Aero-thermal Investigation of Turbine Endwalls and Blades). Such a cascade has a relatively low solidity that is responsible for the formation of a laminar separation bubble on the suction side of the blade. Separated-flow transition and transonic conditions downstream of the throat result in a flow configuration that is very challenging for traditional RANS solvers. Moreover, the separated flow transition pattern was found to have a strong impact on both the aerodynamic and thermal aspects. The study was carried out using a novel three-equation, transition-sensitive, turbulence model. It is based on the coupling of an additional transport equation for the laminar kinetic energy to the Wilcox k - ω model. Such an approach allows one to take into account the increase of the nonturbulent fluctuations in the pretransitional and transitional region. Comprehensive aerodynamic and heat transfer measurements were available for comparison purposes. In particular, heat transfer measurements cover different Mach and Reynolds numbers, in both steady and periodic unsteady inflow conditions. A detailed comparison between measurements and computations is presented, and the impact of transition-related aspects on the surface heat transfer is discussed.