As a type of lepton, muons mainly come from cosmic rays or accelerators. Cosmic ray muons have high energy and strong penetrability, so that they can be regarded as a natural non- destructive fundamental particle “probe” to image and monitor objects. Accelerator-based muons have high intensities and adjustable energy, which allows them to image objects quickly. The negative muons generated by accelerators can form muonic atoms after being captured by nuclei inside materials; thereafter, the muonic atoms can radiate X-rays, from which the elemental composition of the material can be analyzed. This article reviews the basic principles of cosmic- ray muon imaging, accelerator-based muon imaging and muonic X-ray elemental analysis, as well as their major application fields and future developments. In particular, the accelerator-based muon imaging and muonic X-ray elemental analysis projects at the China Spallation Neutron Source will be presented.