[en] In a way, disease could be considered as the result of gene expression deregulation. Knowing what genes are expressed in a particular cell or tissue and how they are regulated allow to better understand disease mechanisms. Transcriptomic is the study of the expression of the entire genome. This technical challenge has been rendered possible as different species’ genomes began to be sequenced. Several techniques have rendered efficient this process of measuring simultaneously the expression level of a large number of genes. To date, micro-array technology is probably the best known. However, with the development of more affordable techniques, sequencing/ resequencing could compete with microarrays in the future. All together, these techniques open new ways for early diagnosis and prognosis for the practioner ‘in the field’.