The unique feature of the seed of tetraploid cotton (Gossypium hirsutum and Gossypium barbadense) is that about 30% of the seed coat epidermal cells develop into cellulose-enriched fibres, while the embryos synthesize oils and proteins. Hence, both the maternal and filial tissues of the cotton seed are of significant economic value. After initiation from the ovule epidermis at or just before anthesis, the single-celled fibres elongate to 2.5–6.0 cm long in the tetraploid species before they switch to intensive secondary cell wall cellulose synthesis. Thus, apart from its agronomic importance, the cotton fibre represents a model single-cell system to study the control of cell differentiation and elongation, carbon partitioning to cellulose synthesis and also the interaction between maternal (fibre) and embryonic tissues in seeds. Over the past decade or so, significant effort has been made to understand the cellular and molecular basis of cotton fibre development and oil biosynthesis in the embryo. Metabolic engineering of the oil biosynthetic pathway in cotton seed has successfully produced healthier and stable oils. A number of candidate genes and cellular processes that potentially regulate various aspects of fibre development have been identified. Further elucidation of the in vivo functions of those candidate genes could significantly deepen our understanding of fibre development and offer potential for improvement of fibre quality through genetic engineering or marker-assisted breeding approaches.