The gene regulatory apparatus that directs development is encoded in the DNA, in the form of organized arrays of transcription factor target sites. Genes are regulated by interactions with multiple transcription factors and the target sites for the transcription factors required for the control of each gene constitute its cis-regulatory system. These systems are remarkably complex. Their hardwired internal organization enables them to behave as genomic information processing systems. Developmental gene regulatory networks consist of the cis-regulatory systems of all the relevant genes and the regulatory linkages amongst them. Though there is yet little explicit information, some general properties of genomic regulatory networks have become apparent. The key to understanding how genomic regulatory networks are organized, and how they work, lies in experimental analysis of cis-regulatory systems at all levels of the regulatory network.