Gene expression is a central concept in molecular biology: its control, frequently exquisite in terms of cell specificity and timing, forms part of our explanation of most biological processes. The importance of the control of gene expression for developmental biologists is made obvious by just considering the nature of their discipline. Development is the term we use to describe the coordination in time and space of numerous cellular activities such as mitosis, migration, differentiation and apoptosis. Understanding the role of genes in these processes thus necessitates the use of methods to determine patterns of transcription during development with a high degree of sensitivity and specificity, conventionally by in situ hybridization. However, there is a widespread conviction amongst biologists that the description of gene expression patterns is of no immediate functional relevance: definitive functional data are the exclusive prerogative of biochemistry and genetics. In this review of recent applications of DNA microarray technology by developmental biologists, we suggest that genome-wide expression profiling has met with some resistance owing to such preconceived ideas about the status of gene expression pattern descriptions and, in particular, the format in which these are delivered by microarrays.