This paper presents an overview of energy harvesting, and describes the methods used to generate electrical power from ambient or waste energy sources and includes; photovoltaic; thermoelectric; piezoelectric; pyroelectric; radio frequency (RF); electromagnetic induction; electrostatic; and capacitive methods. A brief description of the scientific principles, typical application, commercial success, and future prospects is discussed. The paper concludes that photovoltaic energy harvesting is the most commercially successful energy harvesting technology to date, partly due to its high efficiency and power density. However, parallel technology developments in low-power boost and DC-DC power conversion, along with energy storage in electrical double layer supercapacitors, have enabled thermoelectricity, piezoelectricity, and electromagnetic energy harvesting to achieve commercial viability and increasing application success. In future, it is likely that a greater focus will be placed on the integration of these different energy harvesting techniques into one overall system, taking advantage of the individual strengths of each technique.