This review provides a summary of research on the penetration of dyes into natural fibres. Emphasis is placed on the relationship between diffusion and the structural morphology of the fibres. The types of commonly dyed natural fibres, the methods of measurement of diffusion and the data that have been collected by the use of each method are reviewed. The applicability of general theories of dyeing to natural fibres is also examined. Recent developments that have led to the direct observation of penetration pathways in wool fibres are described, together with the insights obtained into diffusion phenomena. Areas for future work are briefly indicated.