In recent years the use of bidirectional DC-DC converters in various applications have been developed. In bidirectional converters, direction of current flow can be changed without changing the polarity voltage sources connected to the converter and power can be transferred between the two sources. The capability to transfer power in both directions makes these converters suitable choices for uninterrupted power supplies, charger and battery discharging, hybrid electric vehicles, fuel cells, photovoltaic systems. Bidirectional converters can be divided into non-isolated bidirectional converters and isolated bidirectional converters. Non-isolated bidirectional converters have disadvantages such as having only one output, no isolation between input and output and having limited gain. At high output voltage to input voltage ratio and where isolation is needed, isolated bidirectional converters should be employed. Due to the leakage inductance in isolated bidirectional converters, switching losses increase and high voltage stress is imposed on the switches. Soft switching auxiliary circuits are used to create soft switching conditions. Providing soft switching for bidirectional converters is much more difficult because the power flows in both directions. To avoid circuit complexity while providing soft switching condition, same auxiliary elements should be used for both power directions. In addition, it is desirable that the auxiliary circuit does not cause noticeable additional losses. In this thesis, bidirectional converters, their applications and employed soft switching technics are reviewed. Then, an isolated soft switching SEPIC-zeta bidirectional DC-DC converter is introduced. This converter has transformer isolation between the input and the output. This converter has two main switches, therefore can be used in low power and medium power applications. The voltage gain of this converter is similar to the buck-boost converter, so it can operate under wide range of input voltages. In SEPIC mode, the converter has an input inductor and the input current is continuous and in zeta mode, the converter has an output inductor which increases the output power density. In the proposed converter, in order to reduce switching losses, an auxiliary circuit is used. The auxiliary circuit has only one auxiliary switch with soft switching performance. In this converter, the auxiliary switch turns on before turning off the main switches and causes the current flow in the main switches to be reduced to zero. Therefore, the voltage stress of switch due to the leakage inductance in isolated bidirectional converters is eliminated. The main feature of the proposed converter is providing soft switching condition for both converter switches with only one auxiliary circuit which has improved the converter efficiency in comparison to the traditional isolated SEPIC-zeta bidirectional DC-DC converter. The proposed bidirectional converter is analyzed for both SEPIC and zeta operating modes. The validity of theoretical analysis is justified using the experimental results of a 100 W, 48–200 V prototype converter. Keywords 1- Switching Power Supplies 2- Isolated Bidirectional Converters 3- Soft Switching 4- SEPIC-zeta