Ion sources are a critical component of all particle accelerators. They create the initial beam that is accelerated by the rest of the machine. This talk will introduce the many ways of making a beam for high power hadron accelerators. A brief introduction to the some of the relevant concepts of plasma physics and beam formation is given. The different types of ion source used in accelerators today are examined. Positive ion sources for producing H+ ions and multiply charged heavy ions are covered. The physical principles involved with negative ion production are outlined and different types of negative ion sources described. Cutting edge ion source technology and the techniques used to develop sources for the next generation of accelerators are discussed.