Over the past decades, the use of plant fibre reinforced composites has increased significantly due to their many attractive attributes such as high specific strength and modulus, wide availability, low cost and high environmental credibility compared to their synthetic counterparts. These attributes are especially attractive for lightweight applications in automotive, marine, aerospace and sporting goods sectors. This growth is expected to continue in the future. To improve the design and performance of bio-based composites, an improved understanding of processing-structure-property relations in such bio-based composites is required, the fibres being the key component of the composite to obtain performing properties. This is due to the sensitivity of the constituent plant fibres to mechanical stress (pressure), temperature, water and other parameters. The purpose of this review is to critically synthesise literature on the impact of composites processing steps on plant fibre cell wall structure and properties. The impact of plant fibre composites processing steps from the polymer impregnation stage right through to the end-of-life recycling stage is reviewed. Additionally, mechanical, morphological and hygroscopic properties of plant fibres are considered in conjunction with process times, temperature and shear rate. This review will aid process and product designers to develop new performing plant fibre composite products, taking into account the process parameters to select the most optimised process and (their effects on) plant fibres. Considering how fibre properties change with biocomposites processing steps is indeed essential to understanding the links between the micro and macro scales, and to be able to design optimised plant fibre composite materials.