Optical tweezers have grown to be one of the most powerful and versatile single-molecule methods for analysis of DNA-protein complexes. The power of optical tweezers lies primarily in its extremely high sensitivity and bandwidth in combination with a wide and biologically relevant force range. To accurately apply and measure a force in an optical tweezers assay, the biological system, a DNA molecule, is tethered on two opposite ends. Most commonly used are the single-beam and dual-beam force measuring optical tweezers. Extreme refinement of optical tweezers sensitivity has led to unprecedented mechanistic insight into the single-base-pair stepping of RNA polymerases during DNA transcription. Experiments on DNA repair, which involves the orchestrated action of many proteins, each with different physical characteristics, have showcased the versatility of optical tweezers assays and also illustrated the large advantages of concurrent visualization by fluorescence microscopy.