Light carries momentum, so an object that reflects or refracts a beam of light experiences a force. This force is very small, but still strong enough to manipulate objects, such as a polystyrene bead. Using light focused through a lens, beads can be “trapped” near the focus. These optical traps, or “optical tweezers” have become an important tool that allows researchers to manipulate individual molecules or molecular complexes. High-resolution optical trapping techniques can now detect movements on the scale of a single base pair of DNA, 3.4 angstroms. The October issue of Cold Spring Harbor Protocols includes a series of articles detailing the concepts behind optical trapping, the components of an optical trapping system and the single-molecule experiments in which they are used. Carlos Bustamante, Yann Chemla, and Jeffrey Moffitt provide an introduction to High Resolution Dual-Trap Optical Tweezers with Differential Detection, and subsequent articles on Managing Environmental Noise, Instrument Design, Data Collection and Instrument Calibration, Minimizing the Influence of Measurement Noise and Alignment of Instrument Components.