A review is given of fundamental information about CW-pumped dye lasers that is most useful to users of these lasers. The important attribute of continuous tunability is discussed and limits suggested for the following: Wavelength ranges, linewidth, mode purity, polarization and power output. The physics of operation of the dye laser is described and rate equations are solved for the dye solution flowing through a focused pump beam. A detailed discussion is given of the design limitations, on efficiency and on wavelength selectivity, that are imposed by considerations of geometrical and physical optics. It is shown that tradeoffs may have to be made between high power output and efficiency, and high wavelength selectivity. Finally, there is brief mention of the chemistry of the dye solution.