For the past 15 years, there has been tremendous interest and technological development concerning biocomposites. Plant fibres can be derived from a multitude of natural agro-sources, with the preferred choice as a composite reinforcement often being driven by abundance, geographical location, and historical use. While from a product designer's or engineer's point of view, all plant cell walls are 'similar', they have indeed substantial morphological and mechanical diversity linked to their structure, biochemical composition and the plant growing conditions. Here, we provide a holistic overview of the main types of plant cell walls used as polymer reinforcements. The relationship between their structures and properties, in constant link with potential associated composite, is specifically discussed. Then, the fibre extraction and cultivation modes are compared, through an environmental assessment. We also show how a scientists point of view on cell wall structure and associated experimental approach lead to distinct results; following a critical review, we make recommendations on appropriate characterisation. A final discussion highlights the pertinent parameters that accurately define a composite reinforcement fibre. The review will serve as a handbook reference for researchers and designers in the field of biomaterials for appropriate selection of plant cell walls for specific composite applications.