The velocity that can be attained by a solar sail is limited by the Sun’s intensity. In this chapter we examine replacing the Sun with a much more intense artificial radiation source, such as a laser, to achieve a higher velocity. We look more closely at the mechanics of photons incident on sails, taking into account relativistic effects and the finite travel time of light. Diffraction of the directed radiation source is a major issue, because it limits the amount of light intercepted by the sail, requires large optics, or demands high laser power, or some combination of these. We look into achieving the kilometre-scale optical systems required using aperture synthesis and adaptive optics. Given the large laser powers involved (tens to hundreds of gigawatts), sail heating becomes a limiting factor, and the choice of materials is critical. Having looked at the theoretical issues, I summarize a few of the numerous laser sail designs that have been published that bring together many of the issues discussed.