Over the last decade, single-molecule techniques have proven their wide applicability in the study of processive motor proteins and other enzymes, yielding insight into their kinetics and mechanochemistry. In the context of force spectroscopy of protein–nucleic acid interactions, optical tweezers, magnetic tweezers, and atomic force microscopy have made important contributions. Advantages of magnetic tweezers include particularly straightforward control of the supercoiled state of DNA, facile extension to parallel measurement of multiple molecules and to integration with fluorescence measurements, and the simplicity and robustness of the experimental configuration. This chapter reviews the principles behind magnetic tweezers and their experimental implementation and points out recent improvements. It also describes several types of experiments that can be performed using magnetic tweezers.