This chapter introduces a lumped modeling and design approach for bending beam type resonant piezoelectric vibration energy harvesters PVEH. The developed framework is used to clarify the achievable power density limits of these devices. Four possible bottlenecks are discussed: the amount of energy transferred to the harvester from the source, the amount of strain energy that the structure can support, what fraction of this energy is converted electrically, and the fraction that is scavenged by the electric load. Due to the simultaneous stiffening and dampening caused by the piezoelectric effect, the resonance frequency shifts with the increase of the impedance of the connected load. In its simplest form, the inertial vibration energy harvester is an electromechanical resonator excited by its own inertia. The actual power output expected from such a device is therefore fundamentally tied to the rate at which it can inertially capture energy from its surrounding.