Over the last decade, composites of polymers reinforced with natural fibres have received increasing attention, both from the academic world and from several industries. There is a wide range of natural fibres which can be applied as reinforcements or fillers thanks to their properties and availability; i.e. flax, hemp, jute, kenaf and sisal. Natural fibres are mainly attractive for the following reasons: specific properties, price, health advantages and recyclability. Particularly, industry is getting more and more interested in environment-friendly products and therefore the research on natural fibre based on composite materials is gaining importance. Some of the benefits linked to the usage of such natural composites are their low density and good specific properties. Furthermore, they are renewable and have a CO2-neutral life cycle, in contrast with their synthetic opponents (i.e. glass and carbon). Despite such good premises, these fibres have also some negative characteristics: they are highly hydrophilic and their properties may vary in time. The natural fibres have a complex structure of elementary fibres, consisting of cellulose, hemicellulose, pectin, lignin and others and thus they should not be considered as monofilament fibres. Mechanical, physical and even chemical properties of these fibres are strongly harvest-dependent, influenced by climate, location, soil characteristics, and weather circumstances. They are affected also by fibre processing (i.e. retting, scutching, bleaching, spinning) and by their incorporation into composites: handling, impregnation and consolidation may introduce supplementary changes. Obviously, such variability complicates the prediction and the evaluation of the composite properties. Another known problem in natural fibre reinforced composites is the poor interface quality between the fibres and the polymer matrix; chemical pre-treatments are often applied in order to enhance the adhesion between these components,. In the last few years several new components based on natural fibre composites have been mainly developed by industries in the automotive field; i.e. in 2003 around 43,000 tonnes of natural fibre have been used by the European automotive industry as composite reinforcement. This interest is due to the excellent tensile mechanical properties of fibre obtained from plants such as flax, hemp and the stinging nettle. Other main applications of the natural fibres are in the following fields: nautical and biomedical ones. The aim of this work is to present a review on the main natural fibres investigated by researchers and to report the results of some original studies performed by the Authors.