We assess different power limits of cladding-pumped fiber lasers. Despite recent advances in pump sources, these are still primarily limited by available pump power. We find that it should be possible to reach output powers beyond 1 kW in single-mode ytterbium doped fiber lasers. Experimentally, we have realized an ytterbium-doped fiber laser with 272 W of output power at 1080 nm, with an M2-value of 3.2, as well as an erbium-ytterbium co-doped fiber laser with 103 W of output power at 1565 nm, with an M2-value of 2.0. We believe these are the highest-power ytterbium and erbium-ytterbium fiber lasers ever reported.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.