This manual describes an advanced undergraduate laboratory experiment in the use of optical tweezers.  The experiment uses a commercially available optical tweezer apparatus based on an infrared diode laser with visible light illumination and quadrant photodetection.  Using this system the student can characterize the strength of the optical trap, observe Brownian motion of a particle in the trap, and explore the biological forces associated with flagellar swimming of bacteria and vesicle transport in a plant cell.  The physics behind the experiment, including the power spectrum of the particle's motion in the trap and the method of trap calibration, are described in detail.