Applications of cosmic-ray muons

Applications of cosmic-ray muons have grown in numbers in the last decades. This was possible thanks to the development of detectors and techniques employed in particle and nuclear physics. Indeed the first famous application, the scanning of the Chephren's pyramid, was performed by L. W. Alvarez, that was a great expert in particle detectors and indeed was awarded a Nobel prize for his work on the hydrogen bubble chambers. After a first period in which the applications exploited mainly the absorption of the cosmic-ray muons when crossing a structure under investigation, more recently also the deflection of the muons has been used to design new applications. Nowadays more and more groups around the world are working on this research field. In the present review, after an introduction on cosmic-ray muons, the principles of the interaction of muons with matter will be briefly summarised. This description is important to classify the applications in three main categories: muon radiography, muon tomography and muon metrology. In the following, for each class, an overview of the basic ideas and a detailed description of the technologies will be presented along with a list of past and present applications