The principle design ideas of thin disk lasers [1] will be illustrated and the advantages for operating different laser-active materials will be explained in detail. So far, the maximum laser cw-output power demonstrated from one single disk is more than 10 kW with an optical to optical efficiency of more than 50 %. Another way to scale the output power is the use of several disks in one laser resonator. Using this possibility, more than 27 kW output power with ten disks in one resonator and with good beam quality have been demonstrated. In this contribution the scaling laws for cw-operation of thin disk lasers will be laid down showing that more than 100 kW extracted output power from one single disk will also be possible. In conclusion, scaling the power per disk and using several disks in one resonator or using additional disks in a laser-amplifier, may result in MW class laser systems with good beam quality.