Energy harvesting from wasted or unused power has been the topic of discussion for a long time. This contribution focuses on harvesting energy from mechanical vibrations using piezoelectric transducers. Three major phases/steps associated with piezoelectric energy harvesting are considered: (i) mechanical-mechanical energy transfer, including mechanical stability of the piezoelectric transducer under large stresses, and mechanical impedance matching, (ii) mechanical-electrical energy transduction, relating with 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. The problem found in the current research teams is on a narrow research area of each above phase. In order to provide comprehensive strategies on how to improve the efficiency of the harvesting system, general guideline for piezoelectric energy harvesting systems is provided.