In principle, a knowledge of the genome of an organism should lead to an understanding of its metabolism. However, things are not so simple. First, genes have to be expressed and when expressed it is important to know to what extent; it is a crucial point in the modelling of metabolism and the study of fluxes. Second, a knowledge of the structure of a gene does not provide information concerning the properties of the corresponding encoded enzyme (Km, regulation, etc.). A first answer to these questions is given by transcriptome analysis using microarrays. It should be noted however that it is not exactly the answer to our questions: RNA expression is not necessarily proportional to protein expression even if this is generally true. A better answer, technically more difficult (2D gels for instance), is provided by proteome analysis at a given time under specific conditions, combined with knowledge of the kinetic properties of the enzymes.