Vibration energy harvesting has got much research interest in recent years as non-exhaustive power source of wireless sensor nodes and wearable electronics. In case of wearable electronics and body sensor network the frequency of vibration produced by human passive motion is as low as 1-10Hz. At such a low vibration frequency harvestable power reduces dramatically, leading researchers to suggest mechanical frequency up-conversion techniques. This paper presents challenges in energy harvesting at low frequencies and overviews the frequency up-conversion techniques suggested in literature. © 2011 IEEE.