Different aspects of barley grain development have been studied in detail, but a more global analysis of gene expression patterns is still missing. We have employed macro arrays, containing 1184 unique sequences from 1421 barley cDNA fragments, to study gene expression profiles in maternal and filial tissues of developing barley caryopses from fertilization to early storage phase. Principle component analysis (PCA) defined distinct expression networks in the pre-storage (0, 2, and 4 days after flowering (DAF)) and early storage phase (10 and 12 DAF). During an intermediate phase (6 and 8 DAF), PCA visualizes a dramatic re-programming of the transcriptional machinery. In maternal tissues, a large set of protein-mobilizing enzyme mRNAs, together with upregulated lipid-mobilizing enzyme and downregulated reactive oxygen species (ROS)-scavenging enzyme genes, suggests mobilization of stored compounds and programmed cell death (PCD). In the filial tissue fraction, a set of genes highly expressed during the pre-storage phase is involved in growth processes, including cell wall biosynthesis. The data suggest that the necessary UDP-glucose is provided both by sucrose synthase (isoform 3) and an invertase-driven pathway. Further, major developmental changes in pathways producing energy are predicted. A bell-shaped expression profile with a peak during the intermediate phase is characteristic for genes associated with photosynthesis and ATP production. The photosynthesis-determined increase of ATP concentration could be a prerequisite for the initiation of grain filling, dominated by starch and storage protein synthesis. Storage product accumulation is accompanied by high transcriptional activity of genes involved in glycolysis and fermentation, as well as in the citric acid cycle.