Living cells carry thousands of nanomachines that assemble, disassemble, transport or process biomolecules. We study machines of gene expression (the path from genes on DNA to functional proteins) by observing single molecules of gene-expression machinery. Our main tool is single-molecule fluorescence spectroscopy, a technique that can measure nanometre distances and study molecular interactions in real time (as "molecular movies"). We also study artificial machines and devices, such as DNA-based nanodevices. Our work is multidisciplinary, combining optics, spectroscopy, biochemistry, molecular biology, molecular modelling and signal processing.