This Ph.D. thesis investigates the relation between microstructural and mechanical properties of Advanced High Strength Steels (AHSS), with the goal of developing a microstructure with optimised mechanical properties. Among different grades of AHSS, Quenching and Partitioning (Q&P) steel which is composed of thin films of retained austenite between carbon depleted martensite laths, is selected. Different Q&P microstructures were developed in a 0.3C-1.6Si-3.5Mn (wt.%) steel with non-homogenous chemical composition. Aiming an adequate control of the microstructural evolution during the Q&P process, the heat treatments were performed on small specimens in the dilatometer. In view of this, the thesis is divided into three parts: chapter 2 investigates the influence of specimen size on the tensile behaviour of steels, chapters 3 and 4 outline the methods to characterize the microstructural properties of the Q&P specimens and chapters 5 and 6 discuss the relation between the mechanical and microstructural properties. Chapter 2 studies the influence of the specimen geometry on the mechanical behaviour of steels. Miniature and standard specimens from different grades of steels were tested in tension. The results show that while the specimen geometry has insignificant influence on the actual elastic strain of the materials, the elastic strain which is measured from the crosshead displacements is higher than the actual strain. The reason is that the elongation of the fillet zones and the machine compliance are recorded along with the specimen elongation as the crosshead displacement. A mathematical model is developed to correct the influence of the elastic strain of the fillet-zones and the machine compliance. For different types of steels, the calculated elastic strain and the strain measured on the standard specimens are in good agreement and consequently the proposed model can be used for calculating the elastic strain of the miniature specimens from the crosshead displacement. Moreover, it was found that the yield strength, ultimate tensile strength and uniform elongation of steels are almost independent of the specimen gauge length. Total elongation increases with decreasing the specimen gauge length. This is a result of the calculation method, since the total elongation is calculated by dividing the elongation of the specimen by the initial gauge length, which is smaller for miniature specimens. Since the post-uniform elongation is independent of the specimen parallel zone, the measured total elongation is higher in miniature specimens. A method was applied for converting the total elongation of the miniature specimen to the total elongation obtained from standard ones. In chapter 3 an improved method is developed to measure dislocation density of a lath martensitic steel by applying X-ray diffraction profile analysis. This was done by combining the modified Williamson-Hall equation (MWH) and modified Warren-Averbach (MWA) methods. The proposed method is independent of limitations due to the considered range of the Fourier length. This method leads to a dislocation density that is in good agreement with the dislocation density determined based on the dislocation strengthening. The MWH method, under the assumption of a fixed value for the dislocation distribution parameter, was applied to calculate the dislocation density. The calculated dislocation densities are in the range of the values determined from the dislocation strengthening. However, it was found that the combined MWH and MWA method can be used as a quantitative method for dislocation density calculations, with a better accuracy than just the MWH method. Chapter 4 investigates microstructural development during application of the Q&P process in a steel with inhomogeneous chemical composition. In place EPMA and SEM analysed show that during the initial quenching, in Mn/C/Si-poor regions higher fractions of initial martensite are formed than in Mn/C/Si-rich regions. This leads to a non-homogenous distribution of initial martensite in the matrix. Lowering the quenching temperature, a higher fraction of austenite transforms to initial martensite and therefore microstructural banding decreases. Moreover, it was found that precipitation of ?-carbides during the first quenching reduces the concentration of carbon in solid solution in martensite. Regarding the fact that the partitioning of carbon present in carbides requires the decomposition of the carbides and in view of slow kinetics of carbide decomposition, full completion of the carbon partitioning process can be achieved only after isothermal holding times longer than predicted by simulations of carbon partitioning. A method was developed to determine carbon concentration of secondary martensite, martensite that is formed during final quenching, on the basis of dilatometry data. Additionally, it was found that at the initial stage of isothermal holding, carbon partitioning stabilizes a certain fraction of austenite. This stable austenite does not decompose to bainite during the isothermal holding and is retained at room temperature. In the specimens with higher quenching temperature, carbon partitioning stabilizes a larger fraction of austenite and therefore a lower fraction of bainite is formed. Furthermore, bainite formation reduces the volume fraction of secondary martensite, formed from unstable austenite, by two mechanisms. First, bainite formation is accompanied by carbon diffusion from bainite to austenite. This results in stabilization of a part of the unstable austenite. Secondly, bainite forms from unstable austenite and consequently decreases the fraction of unstable austenite. Chapter 5 studies the relation between the yield strength and microstructural properties of the constituent phases i.e. retained austenite, initial martensite, bainite and secondary martensite. The in-situ X-ray diffraction analysis showed that there is an insignificant austenite to martensite transformation prior as well as during yielding of steels. Therefore, the induced martensite formation does not have significant influence on the yield strength. Yield strength of initial martensite, bainite and secondary martensite which was estimated by applying physical models are higher than the total yield strength of specimens. The summation of the normalised yield strength of the constitute phases gives an acceptable approximation of the total yield strength. In this matter, the reduction of the yield strength of the Q&P specimens by increasing the quenching temperature could be related to the decrease of the dislocation density of initial martensite. Chapter 6 showed that a good combination of high strength and elongation is obtained by decreasing the quenching temperature which provides a high fraction of initial martensite with high dislocation density. Moreover, microstructures with high fraction of initial martensite have higher fraction of retained austenite as well as low fraction of secondary martensite, as a brittle phase, and therefore show high elongation. Mechanical properties of the developed microstructures can compete with other types of AHSS steels.