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The master alloy (MA) concept was brought up in the early 1970s as a means to introduce alloying elements with high oxygen affinity in low alloy sintered steels. Afterwards, it evolved to become what we currently understand: a MA as a powder that &- admixed in very small amounts &- can be used as a vehicle to efficiently tailor the microstructure and final properties of the steel through alloying effects. One of the main advantages of the MA approach is the unique opportunity of designing the alloy composition to enhance sintering though the formation of a liquid phase with tailored characteristics. This work presents an overview on some critical aspects to consider during the design of a MA composition. Particular emphasis is devoted to the interactions between MA particles and iron base powders which divides the design possibilities into two main families: dissolutive (non-infiltrating) and non-dissolutive (infiltrating) liquids, with significant differences in the final microstructures and dimensional stability achieved in the sintered parts. Moreover, when advanced alloying systems containing Cr, Mn and Si are considered, the study of the interaction with the base powder must also include the chemical reactions occurring during sintering. In this latter case, application of high purity sintering atmospheres might not be enough to avoid the formation of stable oxides, since the source of oxygen is in most cases the iron base powder itself.