ATOMIC vapours can exhibit large optical nonlinearities1. When laser light is tuned in resonance with an atomic transition, the absorption cross-section of the atom can become very large, typically seven orders of magnitude larger than the cross-sectional area of its electron cloud 2. Because of these strong nonlinearities different laser beams can interact with one another in an atomic vapour, even at intensities as low as a few milliwatts per cm2. This raises the question1 of whether atomic vapours can be used as nonlinear optical elements for parallel optical image processing. A well-known example of an all-optical image processor is the optical correlator: laser beams with imprinted images interact in a nonlinear medium to produce a signal beam, the intensity distribution of which is related to the correlation integral of (and hence the degree of similarity between) the input images. Here we demonstrate the use of a caesium-atom vapour as the active medium in such an optical correlator. We show that this system compares favourably with others currently in use particularly with regard to its power requirements.