$mu-e$ conversion is the experimentally most interesting lepton flavor violating process. From a theoretical point of view it is an interesting interplay of particle and nuclear physics. The effective transition operator, depending on the gauge model, is in general described in terms of a combination of four terms of transition operators (isoscalar and isovector, Fermi-like as well as axial vector-like). The experimentally most interesting ground state to ground state transition is adequately described in terms of the usual proton and neutron form factors. These were computed in both the shell model and RPA. Since it is of interest to know the portion of the strength exhausted by the coherent (ground state to ground state) transition, the total transition rate to all final states must also be computed. This was done i) in RPA by explicitly summing over all final states ii) in the context of the closure approximation (using shell model and RPA for constructing the initial state) and iii) in the context of nuclear matter mapped into nuclei via a local density approximation. We found that, apart from small local oscillations, the conversion rate keeps increasing from light to heavy nuclear elements. We also find that the coherent mode is dominant (it exhausts more than 90% of the sum rule). Various gauge models are discussed. In general the predicted branching ratio is much smaller compared to the present experimental limit.
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