Environmental pollutions due to burning fossil fuels, global warming, increasing demand for energy, and other problems associated with fossil fuels have turned energy consumption trend toward exploiting renewable sources of energy like sun, wind and hydrogen. Utilizing solar cells and fuel cells for power generation, either as an independent power source, or as a complimentary source alongside other conventional energy sources, has been developing dramatically in recent years. But the voltage level produced by solar cells and fuel cells is low and not enough to directly supply an inverter. So using a high step-up dc-dc converter is indispensable in such systems. Also, high step-up converters are widely used in uninterruptable power supplies, hybrid electric vehicles, and HID lamps used in automobile headlights. Boost converter is the simplest structure used for increasing the voltage level. But using boost converter in high gain applications would result in some serious problems. So, extensive research has been carried out to improve the boost converter problems and develop novel topologies for high step-up applications. Increasing voltage gain without using large duty cycles, limiting voltage stress across switches and diodes, decreasing current stress of circuit elements, alleviating reverse recovery problems of diodes, decreasing switching and conduction losses, input current ripple cancellation and developing simple structures with low device count are of the main objectives of research dealing with high step-up dc-dc converters. In this thesis, after analyzing different non-isolated structures proposed for high gain applications, three interleaved high step-up dc-dc converters are presented. These converters possess high voltage conversion ratio and reduced voltage stress across switches. Higher power density, lower input current ripple, and low current stress of components are achieved via interleaved structure. Input current ripple of the converters is minimized, which could reduce the size of the input filter. The performance of converters are analyzed, and corresponding theoretical analysis are provided. In addition to simulation results, a laboratory prototype of each converter is implemented, and the theoretical analysis is confirmed by the experimented results. Key Words High step-up converters, Interleaved converters, Coupled inductors, Passive clamp