Scanning probe microscopy (SPM) was originally conceived as a method for measuringatomic-scale surface topography. Over the last two decades, it has blossomed into anarray of techniques that can be used to obtain a rich variety of information aboutnanoscale material properties. With the exception of friction measurements, thesetechniques have traditionally depended on tip-sample interactions directed normal tothe sample's surface. Recently, researchers have explored several effects arising frominteractions parallel to surfaces, usually by deliberately applying a modulated lateraldisplacement. In fact, some parallel motion is ubiquitous to cantilever-based SPM, due tothe tilt of the cantilever. Recent studies, performed in contact, noncontact, andintermittent-contact modes, provide new insights into properties such as structuralanisotropy, lateral interactions with surface features, nanoscale shear stress and contactmechanics, and in-plane energy dissipation. The understanding gained from interpretingthis behavior has consequences for all cantilever-based scanning probe microscopies.