Energy management for wireless sensor nodes and wireless sensor networks differs from classical approaches focusing on the management of the storage only. It depends on the application, the energy income, harvesting technologies and sources. Techniques like maximum power point tracking, voltage and current conversion, energy income forecast andmodeling, consumption profile tuning or power flow control are used to manage the energy efficiently. The implementation requires the modeling of the consumption on the one hand and the energy income on the other. The consumption profile can be based on current, voltage, power and energy measurements or model generated forecasts. The energy income is modeled by the temperature, vibration characteristics, irradiation or wind speed. Because the profiles do not match, an energy buffer and an energy management logic are used to match them together. Analog, discrete and switched control architectures are used to perform the energy management task. This book chapter provides an overview over the techniques used in different areas of a wireless sensor node and possible complexity levels. It shows the interaction of these domains and the integration into a node-wide or network-wide holistic energy management. Using these techniques makes energy harvesting from ambient energy sources feasible. It enables and supports the latest developments in industrial areas, the environmental monitoring domain, the “Internet of Things” or smart dust networks.