The integration of genomics and plant breeding is driven by the increasing availability of sequence resources and by technological developments. The simultaneous measurement of the expression of thousands of genes is possible, and comparisons between contrasting genotypes and/or biological states, as well as within segregating populations has become feasible. In genetical genomics, the merger of genetics and genomics, gene expression profiles are quantitatively assessed within a segregating population, and expression quantitative trait loci (eQTL) can be mapped like classical QTLs. Methods and examples of applications related to genetical genomics will be reviewed, with emphasis on hybridisation-based (microarray) and PCR-based (cDNA-AFLP) techniques. Despite the complexity of the molecular mechanisms underlying its development, the study of the cotton fiber has become a trait of primary interest. Several maps, including QTL maps, have been published, structural and metabolic genes related to fiber initiation or elongation have been identified, and several large EST projects have been developed. In this context, the applicability of a genetical genomics approach for the study of cotton fiber quality will be discussed. A new cooperative project with CIRAD, Bayer CropScience and CSIRO, and supported by the French National Agency for Research, ANR, was initiated in 2007. The project aims at the genetic and genomic dissection of fiber quality using an interspecific Gossypium hirsutum X G. barbadense RIL population. Classical QTL mapping of fiber properties will be undertaken using data from different locations, and eQTLs will be detected using both microarray and cDNA-AFLP population-wide profiling.