Turbulence in the Earth's atmosphere distorts the images produced by ground-based astronomical telescopes. This 'optical turbulence' severely degrades the performance of astronomical telescopes, but its effects can be reduced using adaptive optical correction. The optical turbulence group within CfAI develops and exploits instrumentation to characterise optical turbulence. We have deployed instruments at several major astronomical observatories. The data obtained is used to optimise the design and operation of adaptive optical systems on large telescopes.