Progress in the development and optimization of energy storage and conversion materials necessitates understanding their ionic and electrochemical functionality on the nanometer scale of single grain clusters, grains, or extended defects. Classical electrochemical strategies based on Faradaic current detection are fundamentally limited on the nanoscale. Here, we review principles and recent applications of electrochemical strain microscopy (ESM), a scanning probe microscopy (SPM) technique utilizing intrinsic coupling between ionic phenomena and molar volumes. ESM imaging, as well as time and voltage spectroscopies, are illustrated for several Li-ion cathode and anode materials. Finally, perspectives for future ESM developments and applications to other ionic systems are discussed.