In this paper, we overview the sources of nonlinearities and the methods of analysis for electrostatic (capacitive) vibration energy harvesters by the example of an energy harvester employing a charge pump base conditioning circuit. Electrostatic vibration energy harvesters are devices that contain mechanical resonators driven by ambient vibrations and coupled with conditioning electronic circuits through a capacitive transducer. These devices are characterised by internal and external nonlinearity and complexity and can display irregular behaviour. We give an overview of the capacitive conversion mechanisms and discuss the nonlinear techniques that can be employed for the analysis of nonlinear vibration energy harvesters.