A first principles theory of spin- and angle-resolved photoemission has been developed. The theory is based on density functional theory and is fully relativistic. It is implemented using multiple scattering theory with a Green's function calculated using a real space cluster method. No lattice symmetry or periodicity is assumed and therefore our approach can be applied to low-dimensional systems. We illustrate the theory with a calculation of the photoemission spectra from Ni(1 0 0) and interpret the result in terms of the band structure. The effect of the polarisation of the photon on the spectra is emphasised.