With this in mind, we present and discuss the results on hyperfine fields at positive moons in the ferromagnetic metals Ni, Fe, Co and Gd. In Co and Gd one has to consider - besides the hyperfine field due to the spin polarized interstitial electrons - dipolar contributions from the local magnetic moments of the host. Recent Knight shift measurements in ferromagnetic and paramagnetie Ni allow some conjectures on the origin of the interstitial magnetization. In this contribution we will discuss the application and the behavior of positive muons in metals. We are interested a) in the electronic structure of the implanted point-impurity, and b) in the internal magnetic fields that are present at the muon sites. The first item is a problem of general interest which also applies to the case of hydrogen in metals, the second item is concerned with the magnetic and electronic properties of the host lattice about which one hopes to obtain information by the use of the p SR [muon spin rotation] method I) The attractive features of the applicability of polarized positive muens rest in the facts that muons can be implanted easily in any metal, that it is present in infinite dilution and that it does not possess a complicated electron core. Positive muons, implanted in metals, seem to reside in the interstitial sites in the metbl lattice. They will interact electrostatically with the conduction electrons by which the positive charge is screened over some distance comparable with the radius of a hydrogen atom. There will be a magnetic hyperfine interaction of the Fermi contact type with the conduction electrons. In a non-magnetic metal this will cause the Knight shift. In a ferromagnetic metal, where the conduction electrons are spin polarized, a large hyperfine field at the muon is to be expected in addition to possible dipolar fields from the magnetic ion cores of the host lattice. In e magnetic alloy the internal fields may vary from site to site causing a dephasing of the muon precession. The hyperfine field at a positive muan may most generally be