Modern materials science and biophysics has increasingly focused on studying and controlling intermolecular interactions on the single–molecule level. The peer–reviewed literature contains an increasing number of studies that either measure the interaction forces directly or use mechanical forces to deform the molecules or trigger structural transitions. Molecular force spectroscopy is the result of unprecedented advances in the capabilities of modern force measurement instruments in the past decade and describes a number of techniques that use mechanical force measurements to study interactions between single molecules and molecular assemblies in chemical and biological systems. Examples of these techniques include atomic force microscopy, optical tweezers, surface forces apparatus, and magnetic tweezers. These techniques typically target a specific range of experimental systems and geometries, but all use mechanical force transducers to apply and detect nanonewton range forces between single molecules in condensed phases.