Optical tweezers, or the single-beam optical gradient force trap, is becoming a major tool in biology for noninvasive micromanipulation on an optical microscope. The principles and practical aspects that influence construction are presented in an introductory primer. Quantitative theories are also reviewed but have yet to supplant user calibration. Various biological applications are summarized, including recent quantitative force and displacement measurements. Finally, tantalizing developments for new, nonimaging microscopy techniques based on optical tweezers are included.