Xiang, R. Teichert, J. JACoW Geneva, Switzerland 978-3-95450-187-8 10.18429/JACoW-eeFACT2016-TUT2H6 English 94-99 TUT2H6 ion gun cathode electron SRF Contribution to a conference proceedings 2017 2017-07 https://doi.org/10.18429/JACoW-eeFACT2016-TUT2H6 http://jacow.org/eefact2016/papers/tut2h6.pdf In this presentation the present electron sources and the relevant issues will be discussed. For the electron positron colliders and accelerator based light sources, the electron gun and injector design, are arguably the most critical part. There are a variety of electron source designs: DC guns, normal-conducting RF guns, superconducting RF gun and hybrid guns. All variants have their own ad-vantages and difficulties. We will overview the typical sources around the world, and compare their advantages and main challenges. The polarization production will also be discussed.