The regulation of plant metabolic processes in response to environmental and developmental signals is a complex interaction between optimization of enzyme activity and transcriptional regulation of gene expression. Through painstaking efforts over more than 50 years, many metabolic pathways in plants have been characterized and their regulation investigated, often in detail. Widely available cDNA and oligonucleotide arrays can now be used to analyze the expression profiles of a large number of genes in parallel. Coupling these expression profiles to detailed analyzes of metabolite changes will allow deep insight into the cellular mechanisms of metabolic adaptation to a wide variety of growth conditions.