A boom in genomics and proteomics has prompted a shift towards studying how networks of genes and proteins function on a wider scale. One example of this is the study of behaviours — complex traits that are typically driven by many genes. On page 838, Robert Anholt and Trudy Mackay explain how advances in quantitative genetics and microarray approaches can be used to understand how inputs from multiple genes are integrated to produce behaviours in Drosophila melanogaster.

Exploring the interconnected relationships among genes and proteins is also proving to be fundamental to understanding human genetic disease. On page 811, Choong-Chin Liew and Victor Dzau discuss the genetics of heart failure, highlighting how genomic studies can provide a deeper understanding of this condition by revealing crosstalk between the different pathways involved.

Some of the most exciting recent insights into the complex interactions among biological molecules have come from the expanding discipline of systems biology, from which several fundamental principles of biological systems have already emerged. One of these is biological robustness, explored by Hiroaki Kitano on page 826. Robustness, which is also closely linked with evolvability, is ubiquitous among biological systems, providing them with stability in the face of environmental and genetic disturbances. Insights into how this feature arises are likely to be key to increasing our understanding of biological networks.

A Highlight on page 808 picks up a related theme — it describes an study that provides some of the first insights into how biological systems adapt to the changing requirements of cells. This reveals some striking differences from the properties described for static networks and underlines how much remains to be learned from this relatively new field of biology.