The muon is a charged particle with a spin. It is produced in accelerators by the collision of protons on a target. Carried by a beam line towards the material to be studied, the positive muons are implanted in well-defined sites where their electrostatic energy is minimal. They then interact by Zeeman effect with the local field created by their magnetic environment as in NMR. Each muon decays after a few microseconds, producing, among other things, a positron whose direction of emission is related to the local field. By detecting positrons, we can go back to the local magnetism near the muon. This technique thus allows a detailed study of the properties of matter at the atomic scale. It represents a marriage between the physics of large particle accelerators, quantum physics and exploratory studies in solid state physics.