High-resolution visualization of the internal density composition and its time evolution in volcanoes could help to improve the understanding of underlying volcanic processes and improve the assessment of volcanic hazards. Cosmic-ray muons are elementary particles naturally produced in the atmosphere and are being everywhere on the surface all time. The muons are highly penetrating particles and some of them penetrate through even kilometric-sized volcanic edifices. Muography is an imaging technique that visualizes the density structure in large-scale structures by tracking of the penetrated muons–similarly to how X-raying is utilized for imaging in the human body. The recent progress in development of muon detector technologies has allowed us to operate instrumentation in harsh and varying environments surrounding volcanoes and utilize muography for passive, remote and non-destructive imaging of the internal density structure of volcanoes. In this chapter, we present the principles of muography, the state-of-the-art of instrumentation and imaging techniques, and we highlight recent applications.