In principle, the thin-disk laser concept opens the possibility to demonstrate high power, high efficiency and good beam quality, simultaneously. For this purpose, a very homogeneous pump power distribution on the disk is necessary as well as very low phase distortions of the disk itself. Spatial mode structure and thermal lens effects in an Yb:YAG thin-disk laser have been investigated as function of the pump power in linear and folded resonators. Whereas thermal lens is shown to be very weak due to the thin disk geometry, a strong correlation of the laser mode with respect to the power density distribution of the pump radiation is exhibited. The experimental results are compared with numerical simulations of the field distribution within the resonator as well as in the far field demonstrating the excellent homogeneity of the disk as laser active medium.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.