THE possibility of using an artificial 'guide-star' to measure optical wavefront distortion caused by atmospheric turbulence has been discussed for some time1–4, but few experimental data are available5. Here we report experimental results demonstrating that atmospheric wavefront distortion can be measured by taking fast 'snapshots' of a guide-star formed by light scattered from a laser beam focused in the upper atmosphere. These results agree with a theoretical prediction6 that the mean-square wavefront error created by using an artificial guide-star at a finite distance rather than a real (infinitely distant) star is proportional to the five-thirds power of the telescope aperture. Using this understanding of the physics, we have demonstrated continuous, real-time atmospheric compensation of a 1.5-m telescope using a high-pulse-rate laser, pulse-synchronized wavefront sensor and deformable mirror, and have been able to resolve the 1.3-arcsecond binary star 53 ξ Ursa Majoris in an exposure time of only one second.