Ultrafast thin disk laser oscillators achieve the highest average output powers and pulse energies of any mode-locked laser oscillator technology. The thin disk con- cept avoids thermal problems occurring in conventional high-power rod or slab lasers and enables high-power TEM00 operation with broadband gain materials. Stable and self-starting passive pulse formation is achieved with semi- conductor saturable absorber mirrors (SESAMs). The key components of ultrafast thin disk lasers, such as gain mater- ial, SESAM, and dispersive cavity mirrors, are all used in re- flection. This is an advantage for the generation of ultrashort pulses with excellent temporal, spectral, and spatial proper- ties because the pulses are not affected by large nonlineari- ties in the oscillator. Output powers close to 100 W and pulse energies above 10 µJ are directly obtained without any ad- ditional amplification, which makes these lasers interesting for a growing number of industrial and scientific applica- tions such as material processing or driving experiments in high-field science. Ultrafast thin disk lasers are based on a power-scalable concept, and substantially higher power lev- els appear feasible. However, both the highest power levels and pulse energies are currently only achieved with Yb:YAG as the gain material, which limits the gain bandwidth and therefore the achievable pulse duration to 700 to 800 fs in