Transcriptional enhancers are short segments of DNA (~100 bp to 1 kb) that switch genes on and off in response to intrinsic and external cellular signals. Whole-genome assays suggest that the human genome contains ~400,000 enhancers (~20 per gene). There is emerging evidence that sequence polymorphisms in enhancer DNAs are a major source of population diversity and predilection to disease. My lab uses quantitative live imaging methods to visualize the activities of enhancers that control the development of the early Drosophila embryo. I will discuss transcriptional bursts, coordinate activation of linked genes, and transvection, whereby an enhancer located on one chromosome can activate the expression of a transcription unit located on the other homologue.