In This book, Charpy impact energy of functionally graded steels produced by electroslag remelting been modeled analytically and numerically. To build the models, the yield stress of each graded layer has been related to its density of dislocations through mechanism-based strain gradient plasticity theory and then by assuming Holloman relation for the corresponding stress-strain curve of each layer, tensile strength and tensile strain of the constituent layers were determined via numerical method. In addition, a modification method for taking into account the strain rate has been proposed. The Charpy impact energy of each layer has been related to the corresponding area under modified and un-modified stress-strain curve of that layer and finally by applying the rule of mixtures, the Charpy impact energy of the specimens has been modeled analytically. In addition, finite element simulation has been conducted and the input data are the predicted modified and un-modified stress-strain curve data. All of the proposed models have shown good agreement with the experimental results.