A single beam gradient force optical trap1-3, or “optical tweezers”, exerts forces on microscopic dielectric particles using a highly focused beam of laser light, and can achieve stable, three-dimensional trapping of such particles (for a review, see ref. 4). Using an infrared laser, calibratable forces in the piconewton (pN) range can be easily generated without causing significant damage to living biological specimens. Optical tweezers work through the microscope, without mechanical intrusion within sealed preparations, and can even reach directly inside transparent cells or organelles. Because it is formed by light, an optical trap can be controlled with very high spatial and temporal precision. Its characteristic size (i.e., its “grasp”) is approximately equal to the wavelength of light, but it can be used to capture and/or manipulate objects ranging in size from ∼20 nm to ∼100 mm. Biological preparations (e.g., cells, vesicles, organelles) or small particles (e.g., latex or silica microspheres, perhaps carrying reagents coupled to their surfaces) can be held, maneuvered, or released at will. Already, researchers have begun to contemplate experiments that were practically impossible just a few years ago. Some possibilities include: (1) the sorting and isolation of cells, vesicles, organelles, chromosomes, etc.; (2) the direct measurement of the mechanical properties of cytoskeletal assemblies, membranes, or membrane-bound elements; (3) measurement of the tiny forces produced by mechanoenzymes; (4) establishing cell-cell contacts, or measuring receptor-ligand interactions; (5) studying cellular rheology on the micrometer scale; (6) doing cellular microsurgery, membrane fusion, and building novel cellular (or noncellular) structures; (7) capturing and maintaining fragile biological structures away from vessel surfaces, in order to study them in isolation under optimal viewing conditions; (8) and much more! The principles by which optical tweezers work will be explained, and a videotape illustrating a number of experimental uses will be shown.