High step-up DC-DC converters are vastly applied in renewable energy systems. Due to the growth of global energy demand and environmental problems caused by fossil fuels, use of renewable energy sources is growing. But these kind of sources are expensive and thus, the power-electronic circuits should operate efficiently in order to harvest maximum energy from these sources. One of the main issues of these energy sources like solar cells is their low output voltage. The basic topology in order to step up the voltage, is the conventional boost converter. But this converter can not work under extreme duty cycles. So High step-up DC-DC converters should be used. The main challenges in this area are presenting simple structure, avoiding extreme duty cycles, reducing voltage and current stress of devices, solving reverse recovery problems of diodes, reducing input current ripple and providing soft switching condition for switches and diodes. Soft switching is important because it would reduce switching losses and improve the efficiency. At the beginning of this thesis, non-isolated boost converters are reviewed. Then soft switching and high step up techniques which are employed in these converters are analyzed. Afterwards two high step up soft switching converters are presented. In the first converter, coupled inductors and a capacitor are placed in series with the output in order to increase the voltage gain. To achieve soft switching, an auxiliary circuit is used. Before turning the main switch on, the auxiliary switch turns on and the snubber capacitor of the main switch is discharged in a resonance manner. The auxiliary circuit dose not need the leakage inductance energy to discharge the snubber capacitor, therefore it can provide load-independent soft switching. The second proposed converter is the interleaved tructure of the first converter. In this converter, soft switching is achieved and the input current ripple is reduced greatly. In order to prove theoretical analyses, simulation results are provided at the end of each chapter. To verify the simulation results, experimental results of the first converter is also presented. Finally, the conclusions of this thesis and the recommendations to continue this research are presented. Key Words : High step-up DC-DC converters, Soft switching, Interleaved boost converter, Coupled inductors.