Beam Cooling: Past, Present and Future

To date, four main methods to increase the phase-space density of circulating beams in storage rings are operational: cooling of e+e- -beams by synchrotron radiation, cooling of protons and ions by electrons, stochastic cooling of (anti-)protons and ions, and cooling of special ions by laser light. A fifth method, ionisation cooling of muons, is under intense development. Each of these techniques will be covered in detail in specialised contributions at this workshop. The present paper is intended to introduce the different techniques and put them into perspective: specific features will be outlined, common characteristics will be sketched, and potential future applications will be mentioned.