Cladding-pumped fiber lasers and amplifiers offer a number of unique properties. These enabled the very rapid power-scaling seen in the last few years. However, there are many additional advantages, that enable the power to be combined with exceptional control of the output characteristics. Such sources promise to make a drastic difference in a range of new applications. The figure shows the progress in reported output powers from single mode or nearly diffraction-limited fiber lasers at wavelengths around 1.1µm, which now exceeds 1 kW (see, e.g., [1]). Advances in high-power multimode diode and fiber technology, combined with the inherent power-scalability of cladding-pumped fibers, lie behind this rapid progress. Compared to the telecom technology and especially erbium-doped fiber amplifiers that these high-power fiber lasers have evolved from, the most important differences in the fiber design are the use of double-clad fibers, the much larger core and inner cladding sizes that allow for the launch of high-power, large, pump beams and increases the damage threshold, as well as the use of ytterbium-doping. The high efficiency of low-loss Yb-doped fiber lasers (YDFLs) – over 80% is possible with high-quality, high-purity fabrication – means that even a 1 kW fiber laser generates no more than ~200 W of heat. This exceptionally low heat load simplifies heat-sinking. The overall efficiency is further enhanced by improvements in diode efficiency, which is currently at ~70%.