Energy recovery from wasted or unused power has been a topic of discussion for a long time. In recent years, industrial and academic research units have focused on harvesting energy from mechanical vibrations using piezoelectric transducers. These efforts have provided research guidelines and have brought to light the problems and limitations of implementing piezoelectric transducers. There are three major phases/steps associated with piezoelectric energy harvesting: (i) mechanical-mechanical energy transfer, including mechanical stability of the piezoelectric transducer under large stresses and mechanical impedance matching; (ii) mechanical-electrical energy transduction, related to the electromechanical coupling factor in the composite transducer structure; and (iii) electrical-electrical energy transfer, including electrical impedance matching, such as a DC/DC converter to accumulate the energy into a rechargeable battery. This chapter starts from the historical background of piezoelectric energy harvesting, followed by a brief review of recent research trends by pointing out several misconceptions by the researchers. The main parts deal with step-by-step detailed energy flow analysis in energy-harvesting systems with typical piezoelectrics, lead zirconate titanate (PZTs), in order to provide comprehensive strategies on how to improve the efficiency of the harvesting system. We also introduce a hybrid energy-harvesting system from magnetic field noise using a composite of piezoelectric and magnetostricive materials.