This chapter is focused on an efficiency analysis of vibration energy harvesting systems. Vibration energy harvesting systems convert the ambient kinetic energy of mechanical vibrations into useful electricity. Nowadays, vibration energy harvesting technologies are used in various engineering applications (automotive, transportation, aeronautics, smart buildings and cities, heavy and petrochemical industries, etc.), and the power gain of energy harvesting systems is still improving. Although the levels of ambientmechanical energy are very low, an efficient kinetic energy harvesting technology can be used as an autonomous power supply for modern ultra-low-power electronics, wireless sensing and embedded self-monitoring systems.