: Todays, photovoltaic (PV) systems as popular renewable energy sources have gained numerous applications. In these systems, usually at first a DC-DC converter is needed to increase the low level DC voltage of the PV panel (less than 50V) to the desired high level voltage (200V--400V) suitable for connecting to DC bus voltage. These DC-DC converters are known as high step-up converters and in addition to PV systems, they are needed in fuel cell systems, uninterruptible power supplies and hybrid electric vehicles. The conventional boost converter as the simplest non-isolated step-up converter, must operate at very high duty cycles to provide high conversion ratio which leads to low efficiency due to high conduction and switching losses. In addition, high voltage stress across the converter switch and the output diode is another disadvantage of the boost converter. Hence, it is not suitable for mentioned applications. In addition to limited voltage gain, switching losses and reverse recovery problem of the diodes are another problems in conventional boost converter which limit the converter switching frequency and overall achievable efficiency. In this thesis at first, PV modules and photovoltaic systems are introduced. In chapter II, different techniques for increasing the voltage gain of the converter are discussed. Then a review of various high step-up topologies is presented. In Chapter III , the first proposed converter is introduced. In this chapter, a novel step-up converter with single-switch, single-magnetic core and continuous input current is presented for photovoltaic applications. The proposed converter is obtained by integrating boost and Cuk converters so that only one switch and one magnetic core is utilized to provide a high voltage gain. This topology reduces voltage stress across the converter switch and diodes, and provides higher voltage gain compared with the basic Boost and Cuk converters. Thus, high-quality and low-voltage MOSFET at moderate duty cycles can be used to increase the converter efficiency. In order to provide soft-switching condition for the converter switch, a soft-switching auxiliary circuit as lossless passive snubber is added to the proposed converter in chapter IV. The auxiliary circuit provides soft-switching condition at a wide range of output power. In addition, reverse recovery problem of the output diode is alleviated. In chapter V, an improved Quasi Z-Source (QZS) converter is presented. The proposed converter utilizes a switched capacitor circuit to increase the voltage gain of the converter and decrease the voltage stress of the components. Moreover, soft-switching conditions are provided for the converter switch by using an active auxiliary circuit as zero voltage transition (ZVT) cell. Soft-switching conditions are obtained for the auxiliary switch, too. Hence, very high voltage gain is achieved and the overall efficiency of the converter is increased as well. Furthermore, the reverse recovery problem of all diodes is eliminated. Operating principles of each proposed converter are analyzed and design considerations are provided. In order to validate the theoretical analysis, prototypes of the proposed converters are implemented and the experimental results are exhibited. Key Words: Photovoltaic systems , high step-up converters , soft-switching