Sannibale, F. JACoW Geneva, Switzerland 978-3-95450-180-9 10.18429/JACoW-NAPAC2016-TUB3IO02 English 445-449 TUB3IO02 ion gun cathode electron emittance Contribution to a conference proceedings 2017 2017-01 http://dx.doi.org/10.18429/JACoW-NAPAC2016-TUB3IO02 https://jacow.org/napac2016/papers/tub3io02.pdf An increasing science demand for high-repetition rate (MHz-class) FEL facilities, from IR to X-rays, has been pushing institutions and groups around the world to develop proposals addressing such a need, and some of them have been already funded and are under construction. Such facilities require the development of high-brightness high-repetition rate electron guns, and a number of groups worldwide started to develop R&D programs to develop electron guns capable of operating at this challenging regime. Here we describe the approaches and technologies used by the different programs and discuss advantages and challenges for each of them. A review of the present achievements is included, as well as a brief analysis to understand if the present technology performance is sufficient to operate present and future high repetition rate FEL facilities.