Cotton fibres, or lint, are very long single cells containing almost pure cellulose. The fibres develop in the weeks after flowering from single cells on the surface of the young seed. Each fibre cell is single at first but elongates and develops rapidly, eventually forming the mature cotton fibre. These processes require the ordered expression of genes which mate enzymes, structural proteins and signalling molecules that together determine the properties of the fibre. Domestication has modified fibre development to produce cotton varieties with greatly improved fibre length, strength and quality. However, the selection and breeding of plants with desirable fibre characteristics is slow and expensive. As such, future crop improvement is likely to depend upon genetic engineering and the cotton industry has been a leader in research and commercialisation of transgene technology, with momentous consequences for the agronomic properties of the crop such as insect and herbicide resistance. Whilst fibre improvement has not been the initial focus of the application of DNA technology, research to this end has been vastly accelerated in the last decade, particularly in the USA. The recent formation of the international Cotton Genome initiative between scientists in USA, France and Australia represents a global effort to develop a saturated and fully integrated genetic and physical map of the cotton genome. Gene discovery is also advancing rapidly, with the release of over 20,000 partial gene sequences (called ESTs) from elongating fibres of both diploid and tetraploid cotton, and the use of CDNA microarrays to facilitate global expression profiling.