This article reviews defects in natural fibres and how, ultimately, they affect the properties of composite materials reinforced with such fibres. Under ideal circumstances, certain natural fibre like flax and hemp can display excellent tensile mechanical properties. However, the potential of the fibre is generally not realised in natural fibre-reinforced composites. Partly, this poor performance can be explained by the presence of defects in the fibres known variously as dislocations, kinks or microcompressions. After briefly considering the chemistry and structure of plant fibres, the properties of selected natural fibres are reviewed. The origin of defects and the impact that processing has on their presence is then considered. The effect that defects have on the mechanical properties of bast fibre and their susceptibility to chemical degradation is also reviewed. Finally, the effect that dislocations have on the properties of composites reinforced with natural fibres is discussed and areas of potential further research needed are highlighted.