Abstract: From the spin relaxation/rotation of 100% spin- polarized muons we can obtain the spatial distribution and dynamical fluctuations of internal magnetic fields on the atomic scale. Here we introduce various applications of the technique in condensed matter physics, including how it can be used to obtain unique information about a wide range of phenomena in magnetism and superconductivity, such as magnetic ordering, internal field distribution, spin dynamics with a unique characteristic time window, and how to obtain the electronic configurations in semiconductors. These applications demonstrate that muon spin relaxation/rotation technique is important to comprehend the novel properties and the complex physics of correlated electron systems.