Today's electronics industry faces complex production chains, a high lead density on the circuit boards and high demands on reliability. Thus the quality of the solder joints has become the central issue. The formation of brittle intermetallic compounds (IMCs) in a solder joint during soldering therefore needs to be precisely understood, and the corresponding basic information can be obtained from the phase diagram. To provide the scientific basis for the interactions between Sn-based solders and electroless Ni-Au surfaces, phase equilibria in the ternary Ni-P-Sn system are described in this book. Despite a high number of experimental problems caused by the volatility of phosphorus, a consistent set of phase diagram representations of the Ni-rich part based on the experimental data has been achieved. The phase equilibria are shown as isothermal and isoplethal sections, as well as the liquidus surface and the reaction scheme. In addition, the crystallographic properties of a newly found compound are described and compared to isotypic and other ternary Ni-P-Sn phases. In the light of these new results a number of ambiguities in the existing solder-related literature are addressed.