A theoretical study of radiation from bends in multimode optical fibres is developed on the basis of ray theory, and the results are compared with experimental measurements. The same theory is also used to predict the power in the core of the fibre as a function of angle around the bend. On the basis of the agreement with experiment for the radiated power from multimode fibres, the same technique is then applied to a single mode fibre using a single axial ray to represent this case. A solution is obtained which is close to that previously obtained experimentally, and also close to alternative theoretical solutions.