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Solar energy, considered as abundant and clean renewable energy source, has been utilized for a variety of applications such as generation of electricity for low and medium power. Nowadays, due to the high-scale penetration of photovoltaic systems, reliable and efficient grid-connected photovoltaic (PV) systems using the advances of power electronics and control system technology are desirable. Thus, single-stage gridconnected photovoltaic systems, have gained attention, especially in low voltage applications. However, PV systems exhibit nonlinear behavior due to the intrinsic features of the PV cell and nonlinear switching functions of the inverter that could negatively affect the performance of the system if they are not adequately compensated for. In this dissertation, using the general structure for the synchronous dq0 frame, a single-stage three-phase grid-connected photovoltaic inverter with a nonlinear control strategy is developed to track the maximum power regardless of the atmospheric conditions and to control the active and reactive power without the necessity of an additional power converter. A novel trajectory of the reference current is obtained online taking into account the dynamics of the DC link capacitor and the switching function of the inverter. v Furthermore, due to increased penetration of low-power single-phase PV systems in residential applications, a single-stage single-phase grid-connected PV system with a nonlinear control strategy is proposed in this dissertation. Unlike to the three-phase system, the single-phase system includes a novel method to mitigate the double linefrequency current ripple of the PV array, which is the major drawback of the single-phase PV inverter. Moreover, based on the preceded work, the nonlinear controller is combined with adaptive control to estimate the unknown disturbances that physically could appear in the circuit and affect the performance of the system. Additionally, in this study, the stability of the system and boundedness of signals of the closed-loop system are demonstrated by Lyapunov stability analysis for both the three-phase system and the single-phase system respectively. Simulation results show the effectiveness and robustness of the proposed controllers to track the maximum power and to control the active and reactive power to sudden changes in the atmospheric conditions and changes in the load.