The goal of this research is to investigate electrical circuits for piezoelectric energy harvesting, and proposing an electrical interface to enhance the output power of a piezoelectric energy harvester. Following this is a brief literature review on vibration energy harvesting and piezoelectric energy harvesting electrical circuits. In this thesis, two different structures are utilized to investigate the proposed electrical circuits. The first part of this study is focused on wind energy harvesting of a piezoelectric flag, while the second part is dedicated to vibration energy harvesting of a piezoelectric beam. After the literature review, the electrical equivalent model of a piezoelectric energy harvester used for each structure is presented in Chapter 2. Chapter 3 includes investigating electrical circuits using a piezoelectric flag. The first section of this chapter presents the dynamic modeling of two electrical circuits used for the piezoelectric flag. In the second section of Chapter 3, the experimental setup for investigating and comparing the electrical circuits is explained. The results and discussion of the circuits are offered in the third section of Chapter 3. Chapter 4 includes the study of electrical circuits using a piezoelectric beam. In the first section of this chapter, the dynamic modeling of the electrical circuits used for the piezoelectric beam is reviewed. The second section of Chapter 4 presents the numerical investigation of the electrical circuits. The procedure of the experiments that have been run to verify the practicality of the proposed electrical circuits is explained in the third section of Chapter 4. The fourth section of Chapter 4 presents the results and discussion of the experiments on the piezoelectric beam. Finally, the conclusion including the final results of both flag and beam is presented: the proposed circuits for the piezoelectric energy harvester show to enhance the output power.