The family of Hox gene transcription factors confers anterior-to-posterior positional information during embryonic development of a wide variety of animal species. In vertebrates, as in many invertebrates, these genes are found in clusters on the chromosome. A Hox gene's relative position within its cluster determines the time and place of gene activation during axial elongation in vertebrates. These phenomena are known as temporal and spatial collinearity, respectively. Across the animal kingdom, themes and variations of this strict correlation exist, as suggested by the recent genome analyses of various species and their embryonic Hox gene expression patterns. However, clearly Hox cluster architecture and gene position inside the complexes have important implications for the transcriptional regulation of this gene family in various developmental contexts.