Prompted by environmental legislation and citizens’ awareness induced by global warming effects, the market for plant-fibre reinforced composites has been growing steadily for the past 10–20 years, as observed by the substantial increase in academic and industrial research developments. However, the transition to larger production still requires several uncertainties to be overcome. Among these uncertainties, defects in plant fibres are known to decrease the mechanical properties at the composite scale. It is therefore of interest to better understand the defects nature, origin and consequences at the fibre scale to monitor the use of plant fibres as reinforcement. In recent decades, finite element modelling has emerged in various scientific fields as an interesting tool that complements experimental characterization. Finite element modelling is even more critical for small and intricate elements such as plant fibres where standard mechanical tests require substantial adjustments and investments due to their complex ultrastructure compared to synthetic materials. The main objective of this review is to provide a novel overview of defects found in plant fibres and their influence on the mechanical properties of plant fibres based on experimental and modelling work. Through a top-down and multi-scale approach, we first describe the flax fibre ultrastructure with a focus on defects. Then, advanced testing methods and emerging numerical approaches that capture the complex mechanical behaviour of plants