The monopile is the prevailing foundation concept applied in offshore wind farms, mainly due to construction of offshore wind farms in relatively shallow water. The monopile can provide a technically and financially feasible solution in shallow water environment due to its’ ability to support the wind generator ensuring at the same time the maximum production, since it can provide a foundation concept which ensures that the displacement and rotation thresholds set by the wind turbine manufacturers are being met. However, the trend nowadays is to develop wind farms in deeper water and to use wind turbines of higher capacity in order to increase the electricity production. This can be translated to a demand to increase the dimensions of the monopile, leading to complications in the transportation and installation procedures. This thesis project focuses on the possibility to enhance the structural and dynamic behavior of the monopile by filling it with sand. The effect of the sand-fill on the local buckling, on the static displacement, on the natural frequency and on the damping ratio of the structure have been investigated. In order to estimate the effect of sand-fill on local buckling, its’ effect on the ovalization of the monopile’s cross-section was estimated using the relevant European Standards and Dutch guidelines. The analysis has shown that for an increasing Youngs Modulus (E) of the sand-fill, the ovalization of the monopile at the critical cross-section decreases significantly, which in turn increases the bending resistance of the filled monopile. The effect of the sand-fill on the static displacement has been estimated by modeling the monopile in Plaxis 3D. The effect of sand-fill appears to be more significant for lower static loads. The effect of sand-fill on the fundamental natural frequency of the structure was estimated analytically by using the approximate normal modes obtained by the Euler Bernoulli beam equations and numerically, using the finite difference method to model the structure. The results of the analysis have shown that the total effect of the sand-fill on the natural frequency of the structure is negative, and sand-fill should be applied only in cases when a decrease of the natural frequency of an existing structure is required. The effect of sand-fill on the damping ratio of the structure has been performed through Free Vibration Tests which have been simulated in Plaxis 3D. The analysis has shown that the hysteretic behavior of the sand-fill leads to an increase of the damping ratio of the structure. The beneficial effect of the sand-fill on the damping ratio increases as the initial displacement applied at the top of the structure increases. Finally, a financial analysis was performed in order to identify the influence of sand-fill on the foundation cost. This additional cost sets the minimum threshold to be exceeded by the possible financial benefits achieved due to the higher damping ratio, in order that the sand-fill can be applied as a financially feasible solution.